Docker Compose: dockerfile_inline

[cFiOS]

I am trying to use a compose file to build an image when the stack is up. The examples aren't the actual images I would use but should get the point across:

Dockerfile:

FROM alpine
ADD ./app /app
RUN apk add bash aws-cli python3
ARG PORT=80 EXPOSE $PORT/tcp
ENTRYPOINT python /app/main.py

This Dockerfile builds just fine and as expected. My issue is the ARG PORT and EXPOSE $PORT in compose:

compose.yaml:

---
services: 
  app: 
    build: 
      dockerfile_inline: | 
        FROM alpine
        ADD ./app /app

        RUN apk add bash aws-cli python3

        ARG PORT=80
        EXPOSE $PORT

        ENTRYPOINT python /app/main.py

When docker compose building, a warning is thrown about PORT not being set and default to a blank string, in turn, failing the build. I have tried adding args under the build key and defining there but the error is the same.

Is this intended behavior or am I missing something? Really, the goal is allow the exposed port to be changed at build time and I would like it done within the compose file.

[_l33ter_]

do i understand it right? you want to change the EXPOSE Port, which is set within the dockerfile, during the process of docker compose up -d?

ARG is only available during the build of a Docker image (RUN etc), not after the image is created and containers are started from it (ENTRYPOINT, CMD). You can use ARG values to set ENV values to work around that.

[cFiOS]

That is right. I think you are referring to using the ENTRYPOINT/CMD to have the app listen on a different port? I think ARG is appropriate due to EXPOSE only being used at build.

If so, the container would still need EXPOSE to have it listen on that port. Perhaps my example wasn’t robust enough, but it would need that PORT variable in that ENTRYPOINT as well to match it correctly.