Reference Environment Variables in Docker Startup Script

In my previous post, Waiting for Docker Service Container Port to Be Ready, I showed how I delayed container startup until another service was ready to accept TCP connections.

In this post, I build on that approach and make it more reusable by adding environment variables to both the Dockerfile and the docker-compose.yml.

Here’s the updated Dockerfile:

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# [Choice] .NET Core version: 5.0, 3.1, 2.1
ARG VARIANT=3.1
ARG DB_SERVICE=db
ARG DB_SERVICE_PORT=1433
ARG PROJECT=Business.Web

# create a base runtime image with node
FROM mcr.microsoft.com/dotnet/core/aspnet:${VARIANT} AS runtime
EXPOSE 80
EXPOSE 443
RUN curl -sL https://deb.nodesource.com/setup_10.x |  bash -
RUN apt-get install -y nodejs

# create a base SDK image with node
FROM mcr.microsoft.com/dotnet/core/sdk:${VARIANT} as sdk
RUN curl -sL https://deb.nodesource.com/setup_10.x |  bash -
RUN apt-get install -y nodejs

# copy source files
FROM sdk as build
COPY src/ ./src

# restore nuget packages
WORKDIR /src
RUN dotnet restore

# run publish command
FROM build as publish
RUN dotnet publish -c Release -o /release --no-restore

# create release image from base runtime image
FROM runtime AS release
# https://github.com/moby/moby/issues/37345#issuecomment-400245466
ARG PROJECT
ENV DB_SERVICE=${DB_SERVICE}
ENV DB_SERVICE_PORT=${DB_SERVICE_PORT}
ENV ASSEMBLY=${PROJECT}.dll
COPY --from=publish /release .
COPY entrypoint.sh .
COPY testconnection.sh .
RUN chmod +x ./entrypoint.sh
RUN chmod +x ./testconnection.sh

CMD ["sh", "-c", "./entrypoint.sh $DB_SERVICE $DB_SERVICE_PORT dotnet $ASSEMBLY"]

And the docker-compose.yml file:

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# https://docs.docker.com/compose/compose-file/compose-file-v3/
version: '3'

services:
  app:
    build: 
      context: .
      dockerfile: Dockerfile
      args:
        # [Choice] Update 'VARIANT' to pick a .NET Core version: 2.1, 3.1, 5.0
        VARIANT: 3.1
        DB_SERVICE: db
        DB_SERVICE_PORT: 1433
        PROJECT: Business.Web
    image: business_web
    environment:
      - ASPNETCORE_ENVIRONMENT=Testing
      - ASPNETCORE_URLS=http://+:80
      - ASPNETCORE_ConnectionStrings__Db=Server=db;Database=Business.Web;User ID=sa;Password=V3ry$ecureP@ssw0rd;MultipleActiveResultSets=False;Connection Timeout=30;
      - DB_SERVICE=db
      - DB_SERVICE_PORT=1433
# https://docs.docker.com/compose/startup-order/
    depends_on:
      - db
    restart: on-failure
    ports:
      - "8080:80"
      - "8443:443"
    volumes:
      - ~/.aspnet/https:/https:ro    

  db:
    image: mcr.microsoft.com/mssql/server:2019-latest
    restart: unless-stopped
    environment:
      - SA_PASSWORD=V3ry$ecureP@ssw0rd
      - ACCEPT_EULA=Y

With this setup, I can create multiple image variations with the same Dockerfile by supplying the three arguments:

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        DB_SERVICE: db
        DB_SERVICE_PORT: 1433
        PROJECT: Business.Web

And when I run the Docker image, I can supply the same three environment variables to override the container defaults. That gives me one Dockerfile I can reuse across different image variations.

Credits:
Persisting ENV and ARG settings to all later stages in multi-stage builds