1~
2Docker aendrede maden vi bygger, leverer og korer software pa. I stedet for "det virker pa min maskine" garanterer Docker, at din applikation korer pa samme made overalt - pa din laptop, pa en kollegas maskine, i CI/CD og i produktion. I denne guide gar vi fra nul til at deploye en rigtig applikation.
3~
4## Hvad er Docker?
5~
6Docker er en platform, der pakker din applikation og alle dens afhaengigheder ind i en standardiseret enhed kaldet en **container**. En container er en isoleret, letvaegtproces, der deler vaertsmaskinens OS-kerne, men har sit eget filsystem, netvaerk og procesrum.
7~
8```mermaid
9graph TD
10    subgraph "Traditional Deployment"
11        A1[App 1] --> OS1[Guest OS]
12        A2[App 2] --> OS2[Guest OS]
13        OS1 --> HV[Hypervisor]
14        OS2 --> HV
15        HV --> HW1[Hardware]
16    end
17~
18    subgraph "Docker Deployment"
19        B1[App 1] --> D1[Container]
20        B2[App 2] --> D2[Container]
21        D1 --> DE[Docker Engine]
22        D2 --> DE
23        DE --> HW2[Hardware]
24    end
25```
26~
27### Containere vs Virtuelle Maskiner
28~
29| Aspekt | Containere | Virtuelle Maskiner |
30|--------|-----------|-------------------|
31| **Opstart** | Sekunder | Minutter |
32| **Storrelse** | MB | GB |
33| **OS** | Deler vaertskerne | Fuldt gaeste-OS |
34| **Isolering** | Procesniveau | Hardwareniveau |
35| **Ydeevne** | Naesten nativ | Overhead fra hypervisor |
36| **Taethed** | Hundredvis per vaert | Tiere per vaert |
37~
38## Installation af Docker
39~
40```bash
41# macOS
42brew install --cask docker
43~
44# Ubuntu/Debian
45curl -fsSL https://get.docker.com | sh
46sudo usermod -aG docker $USER
47~
48# Verify installation
49docker --version
50docker run hello-world
51```
52~
53## Kernebegreber
54~
55### Images
56~
57Et image er en skrivebeskyttet skabelon med instruktioner til at oprette en container. Taenk pa det som et snapshot af din applikation og dens miljo.
58~
59```bash
60# Pull an image from Docker Hub
61docker pull node:20-alpine
62~
63# List local images
64docker images
65~
66# Remove an image
67docker rmi node:20-alpine
68```
69~
70### Containere
71~
72En container er en korende instans af et image. Du kan oprette, starte, stoppe og slette containere.
73~
74```bash
75# Run a container
76docker run -d --name my-app -p 3000:3000 node:20-alpine
77~
78# List running containers
79docker ps
80~
81# List all containers (including stopped)
82docker ps -a
83~
84# Stop a container
85docker stop my-app
86~
87# Remove a container
88docker rm my-app
89~
90# View logs
91docker logs my-app
92~
93# Execute a command inside a running container
94docker exec -it my-app sh
95```
96~
97## Skrivning af en Dockerfile
98~
99En Dockerfile er en tekstfil med instruktioner til at bygge et image. Hver instruktion opretter et lag.
100~
101### Grundlaeggende Dockerfile til en Node.js App
102~
103```dockerfile
104# Use an official Node.js runtime as base image
105FROM node:20-alpine
106~
107# Set working directory
108WORKDIR /app
109~
110# Copy package files first (better caching)
111COPY package.json package-lock.json ./
112~
113# Install dependencies
114RUN npm ci --only=production
115~
116# Copy application code
117COPY . .
118~
119# Expose the port the app runs on
120EXPOSE 3000
121~
122# Command to run the application
123CMD ["node", "server.js"]
124```
125~
126### Byg og Kor
127~
128```bash
129# Build the image
130docker build -t my-node-app .
131~
132# Run the container
133docker run -d -p 3000:3000 my-node-app
134~
135# Visit http://localhost:3000
136```
137~
138## Multi-Stage Builds
139~
140Multi-stage builds holder dine produktionsimages sma ved at adskille buildmiljoet fra runtime.
141~
142```dockerfile
143# Stage 1: Build
144FROM node:20-alpine AS builder
145WORKDIR /app
146COPY package.json package-lock.json ./
147RUN npm ci
148COPY . .
149RUN npm run build
150~
151# Stage 2: Production
152FROM node:20-alpine AS runner
153WORKDIR /app
154COPY --from=builder /app/dist ./dist
155COPY --from=builder /app/node_modules ./node_modules
156COPY --from=builder /app/package.json ./
157EXPOSE 3000
158CMD ["node", "dist/server.js"]
159```
160~
161Dette producerer et image med kun den kompilerede output og produktionsafhaengigheder - ingen kildekode, ingen udviklingsafhaengigheder, ingen buildvaerktojer.
162~
163### Next.js Multi-Stage Eksempel
164~
165```dockerfile
166FROM node:20-alpine AS deps
167WORKDIR /app
168COPY package.json package-lock.json ./
169RUN npm ci
170~
171FROM node:20-alpine AS builder
172WORKDIR /app
173COPY --from=deps /app/node_modules ./node_modules
174COPY . .
175RUN npm run build
176~
177FROM node:20-alpine AS runner
178WORKDIR /app
179ENV NODE_ENV=production
180COPY --from=builder /app/public ./public
181COPY --from=builder /app/.next/standalone ./
182COPY --from=builder /app/.next/static ./.next/static
183EXPOSE 3000
184CMD ["node", "server.js"]
185```
186~
187## Volumes: Persistent Data
188~
189Som standard gar data inde i en container tabt, nar containeren fjernes. Volumes loser dette.
190~
191```bash
192# Create a named volume
193docker volume create my-data
194~
195# Run with a volume
196docker run -d -v my-data:/app/data my-app
197~
198# Bind mount (map host directory to container)
199docker run -d -v $(pwd)/data:/app/data my-app
200~
201# List volumes
202docker volume ls
203```
204~
205## Netvaerk
206~
207Docker opretter isolerede netvaerk, sa containere kan kommunikere.
208~
209```bash
210# Create a custom network
211docker network create my-network
212~
213# Run containers on the same network
214docker run -d --name api --network my-network my-api
215docker run -d --name db --network my-network postgres:16
216~
217# Containers can reach each other by name
218# From "api" container: postgres://db:5432
219```
220~
221```mermaid
222graph LR
223    subgraph "my-network"
224        API[api container\nport 3000] -- "db:5432" --> DB[db container\nport 5432]
225    end
226    User -- "localhost:3000" --> API
227```
228~
229## Docker Compose
230~
231Docker Compose lader dig definere og kore multi-container applikationer med en enkelt YAML-fil.
232~
233### docker-compose.yml
234~
235```yaml
236services:
237  api:
238    build: ./api
239    ports:
240      - "3000:3000"
241    environment:
242      - DATABASE_URL=postgres://user:pass@db:5432/mydb
243    depends_on:
244      - db
245~
246  db:
247    image: postgres:16-alpine
248    environment:
249      - POSTGRES_USER=user
250      - POSTGRES_PASSWORD=pass
251      - POSTGRES_DB=mydb
252    volumes:
253      - pgdata:/var/lib/postgresql/data
254    ports:
255      - "5432:5432"
256~
257  redis:
258    image: redis:7-alpine
259    ports:
260      - "6379:6379"
261~
262volumes:
263  pgdata:
264```
265~
266### Kommandoer
267~
268```bash
269# Start all services
270docker compose up -d
271~
272# View logs
273docker compose logs -f
274~
275# Stop all services
276docker compose down
277~
278# Rebuild and restart
279docker compose up -d --build
280~
281# Scale a service
282docker compose up -d --scale api=3
283```
284~
285## .dockerignore
286~
287Ligesom `.gitignore` forhindrer denne fil unoedvendige filer i at blive kopieret ind i imaget.
288~
289```plaintext
290node_modules
291.git
292.env
293*.md
294.next
295dist
296coverage
297```
298~
299## Bedste Praksis for Produktion
300~
301### 1. Brug Sma Basisimages
302~
303```dockerfile
304# Bad: 1GB+
305FROM node:20
306~
307# Good: ~180MB
308FROM node:20-alpine
309```
310~
311### 2. Kor Ikke som Root
312~
313```dockerfile
314FROM node:20-alpine
315RUN addgroup -S app && adduser -S app -G app
316USER app
317WORKDIR /home/app
318COPY --chown=app:app . .
319```
320~
321### 3. Brug Specifikke Image Tags
322~
323```dockerfile
324# Bad: can change unexpectedly
325FROM node:latest
326~
327# Good: pinned version
328FROM node:20.11-alpine3.19
329```
330~
331### 4. Udnyt Build Cache
332~
333Ordne dine Dockerfile-instruktioner fra mindst til mest hyppigt aendret:
334~
335```dockerfile
336FROM node:20-alpine
337WORKDIR /app
338~
339# These change rarely - cached
340COPY package.json package-lock.json ./
341RUN npm ci --only=production
342~
343# This changes often - not cached
344COPY . .
345```
346~
347### 5. Sundhedstjek
348~
349```dockerfile
350HEALTHCHECK --interval=30s --timeout=3s --retries=3 \
351  CMD wget -qO- http://localhost:3000/health || exit 1
352```
353~
354### 6. Brug Miljovariabler
355~
356```dockerfile
357ENV NODE_ENV=production
358ENV PORT=3000
359```
360~
361## Snydeark for Almindelige Docker-Kommandoer
362~
363```bash
364# Images
365docker build -t name:tag .        # Build image
366docker images                      # List images
367docker rmi image_name              # Remove image
368docker image prune                 # Remove unused images
369~
370# Containers
371docker run -d -p 3000:3000 image   # Run detached
372docker ps                          # List running
373docker stop container_name         # Stop
374docker rm container_name           # Remove
375docker logs -f container_name      # Follow logs
376docker exec -it container sh       # Shell into container
377~
378# Compose
379docker compose up -d               # Start services
380docker compose down                # Stop services
381docker compose logs -f             # Follow all logs
382docker compose ps                  # List services
383~
384# Cleanup
385docker system prune -a             # Remove everything unused
386```
387~
388## Fra Docker til Kubernetes
389~
390Docker handterer individuelle containere. Nar du skal orkestrere hundredvis af containere pa tvaers af flere servere, har du brug for Kubernetes. Docker og Kubernetes er komplementaere:
391~
3921. **Docker**: bygger og korer containere
3932. **Kubernetes**: orkestrerer containere i stor skala (planlaegning, skalering, selvhelbredelse)
394~
395Hvis du er interesseret i naeste trin, kan du se min artikel om Introduktion til Kubernetes.
396~
397## Konklusion
398~
399Docker er en grundlaeggende faerdighed for moderne udviklere. Det eliminerer miljoinkonsekvenser, forenkler deployment og er fundamentet for containerorkestrering med Kubernetes. Start med en simpel Dockerfile, ga videre til Docker Compose for multi-service apps, og adopter multi-stage builds og sikkerheds-bedste-praksis efterhanden som du vokser.
400~
401Den bedste made at laere Docker pa er at containerisere et projekt, du allerede arbejder pa. Start i dag.
402~