Docker Basics for .NET

1. The Problem Docker Solves

Without Docker: "It works on my laptop." Prod server: missing .NET 8, wrong OS, different paths. Crash.
With Docker: Ship your app + .NET 8 + Ubuntu in one container. Server just needs Docker. Runs identical everywhere.

2. Dockerfile - 4 Lines That Matter

Create Dockerfile in project root:

# 1. Build stage: SDK has compiler
FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build
WORKDIR /src
COPY *.csproj .
RUN dotnet restore
COPY . .
RUN dotnet publish -c Release -o /app

# 2. Runtime stage: Only runtime, 100MB not 1GB
FROM mcr.microsoft.com/dotnet/aspnet:8.0
WORKDIR /app
COPY --from=build /app .
ENTRYPOINT ["dotnet", "MyApp.dll"]

Key: Multi-stage. Build with SDK. Run with ASPNET runtime. Final image ~200MB not 1.2GB.

3. Run It Locally - 3 Commands

docker build -t myapp .
docker run -d -p 8080:8080 --name myapp myapp
curl http://localhost:8080/weatherforecast

-p 8080:8080 = Map host port 8080 → container port 8080. Set in launchSettings.json: "ASPNETCORE_URLS": "http://+:8080"

4. docker-compose - Run App + DB Together

docker-compose.yml

services:
  web:
    build: .
    ports: ["8080:8080"]
    environment:
      - ConnectionStrings__Default=Server=db;Database=App;User=sa;Password=YourStrong@Passw0rd
    depends_on: [db]
  db:
    image: mcr.microsoft.com/mssql/server:2022-latest
    environment:
      - ACCEPT_EULA=Y
      - MSSQL_SA_PASSWORD=YourStrong@Passw0rd
    ports: ["1433:1433"]

Run: docker-compose up -d. App + SQL Server up in 20s. Destroy: docker-compose down -v.

1. The Interview Theory: Images vs Containers

Image: Read-only template. Like a class. Built from Dockerfile. Stored in registry. myapp:1.0

Container: Running instance of image. Like an object. Has writable layer. docker run myapp:1.0

Layer Caching: Docker reuses unchanged layers. Put COPY *.csproj + RUN dotnet restore first. If code changes but packages don't, restore layer cached. Build = 5s not 2min.

2. The #1 Interview Question: "Optimize Dockerfile"

Your Answer: "5 rules. 1. Multi-stage: SDK build, ASPNET runtime. Cuts 1GB → 200MB. 2. Layer cache: Copy csproj + restore before code. 3. .dockerignore: Exclude bin/, obj/, .git. Reduces context size. 4. Non-root user: USER app after RUN adduser app. Security. 5. HEALTHCHECK: HEALTHCHECK CMD curl -f http://localhost:8080/healthz. K8s uses it. Also set ASPNETCORE_URLS=http://+:8080 because containers don't run as admin."

3. Production Dockerfile - Staff Level

FROM mcr.microsoft.com/dotnet/aspnet:8.0 AS base
WORKDIR /app
EXPOSE 8080
ENV ASPNETCORE_URLS=http://+:8080

FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build
WORKDIR /src
COPY ["MyApp/MyApp.csproj", "MyApp/"]
RUN dotnet restore "MyApp/MyApp.csproj"
COPY . .
WORKDIR "/src/MyApp"
RUN dotnet build "MyApp.csproj" -c Release -o /app/build

FROM build AS publish
RUN dotnet publish "MyApp.csproj" -c Release -o /app/publish /p:UseAppHost=false

FROM base AS final
WORKDIR /app
COPY --from=publish /app/publish .
# Run as non-root
RUN adduser --disabled-password --home /app --gecos '' appuser && chown -R appuser /app
USER appuser
ENTRYPOINT ["dotnet", "MyApp.dll"]

Why this wins: 1. UseAppHost=false = no extra exe. 2. Non-root = CVE mitigation. 3. Explicit WORKDIR = no path bugs. 4. EXPOSE 8080 = documentation.

4. Environment Variables + Secrets

Dev: docker run -e ASPNETCORE_ENVIRONMENT=Development
Prod: -e ConnectionStrings__Default="..."
K8s: Use secrets: kubectl create secret generic db-secret --from-literal=conn="..."

Code: builder.Configuration.AddEnvironmentVariables(). Double underscore __ = nested JSON.

5. Debugging Containers