consider changing the Docker image to capture stdout

[fdiblen]

Currently it is not possible to use the stdout of Docker image in a GitHub action. This would be useful if one wants to do some tests based on stdout but not only exit codes.

I was able to do that by making the changes below.

1- add `docker-entrypoint.sh`` to set action output variable

#!/bin/sh
set -e

echo "arguments: $@"
step_stdout=$(exec cffconvert "$@")
echo $step_stdout
echo "::set-output name=step_stdout::$step_stdout"

2- modify the Dockerfile to use entrypoint shell script

FROM python:3.9-alpine
RUN apk --no-cache add build-base
RUN python3 -m pip install cffconvert==2.0.0
WORKDIR /app
COPY ./docker-entrypoint.sh /app/
ENTRYPOINT ["/app/docker-entrypoint.sh"]

The changes I made in the GitHub action can be seen here:
https://github.com/citation-file-format/cffconvert-github-action/compare/livetest-stdout

You can see an job with stdout here:
https://github.com/citation-file-format/cffconvert-github-action/runs/4922731817?check_suite_focus=true

[jspaaks]

My preference would be to first investigate if we can make these changes in the github action, as opposed to changing what's in the docker image