Is there a way to print ARGs values, that are passed via the --build-arg flags to a docker build command?
Just using RUN echo $ARG_NAME isn't enough since I want it to be printed before the FROM section, where it's not allowed.
The point is to see these values immediately so I can stop the build quickly, preventing downloading the wrong base images.
Already searched docker docs and google. Maybe someone here can shed some light.
You could use a multi-stage build where the first stage is just for diagnostics and otherwise gets completely ignored.
FROM busybox
ARG ARG_NAME
RUN echo $ARG_NAME
FROM python:3.8
ARG ARG_NAME
...
CMD ["my_app"]
Note that Docker layer caching can cause RUN steps to be totally skipped so even this isn't 100% reliable.
This is not possible
A Dockerfile must begin with a
FROMinstruction
see https://docs.docker.com/engine/reference/builder/
The execution of the RUN command will be performed in an intermediate container that has to be created of an image.