ONBUILD cannot access PYTHON build arg

[ethho]

Summary

Dockerfile ONBUILD requirements installation fails when building a child image:

FROM tacc/reactors:0.8.0
ADD example.json /example.json

Raises:

$ tapis actors deploy -R --debug
Building mwvaughn/consort-chart-manager:0.0.1
The command '/bin/sh -c ${PYTHON} -m pip install -r /tmp/requirements.txt' returned a non-zero code: 127
Traceback (most recent call last):
  File "/Users/mwvaughn/opt/anaconda3/lib/python3.8/site-packages/cliff/app.py", line 401, in run_subcommand
    result = cmd.run(parsed_args)
  File "/Users/mwvaughn/opt/anaconda3/lib/python3.8/site-packages/cliff/display.py", line 115, in run
    column_names, data = self.take_action(parsed_args)
  File "/Users/mwvaughn/src/TACC-Cloud/tapis-cli/tapis_cli/commands/taccapis/v2/actors/deploy/deploy.py", line 181, in take_action
    self._build(parsed_args)
  File "/Users/mwvaughn/src/TACC-Cloud/tapis-cli/tapis_cli/commands/taccapis/v2/actors/deploy/deploy.py", line 256, in _build
    result = self.dockerpy.images.build(path=self.working_dir,
  File "/Users/mwvaughn/opt/anaconda3/lib/python3.8/site-packages/docker/models/images.py", line 287, in build
    raise BuildError(chunk['error'], result_stream)
docker.errors.BuildError: The command '/bin/sh -c ${PYTHON} -m pip install -r /tmp/requirements.txt' returned a non-zero code: 127

Source

Docker has no context for the ARG PYTHON at build time for the child image.

[ethho]

Fixed on 2c0a018.

Always manually test this use case, and brainstorm ways to incorporate it (cleanly) into Actions CI.