fix(docker): update path to CLI jar

[themightychris]

The current :latest Docker container build of the master branch is broken due to changes in #1229:

$ docker pull ghcr.io/mobilitydata/gtfs-validator:latest
latest: Pulling from mobilitydata/gtfs-validator
1efc276f4ff9: Already exists 
a2f2f93da482: Already exists 
12cca292b13c: Already exists 
69e15dccd787: Already exists 
4f4fb700ef54: Already exists 
Digest: sha256:b796fee1fc8e99d857ab1fd6f6438af7d20c3dba74af4d1934f609c22e4dc1ac
Status: Downloaded newer image for ghcr.io/mobilitydata/gtfs-validator:latest
ghcr.io/mobilitydata/gtfs-validator:latest

$ docker run --rm ghcr.io/mobilitydata/gtfs-validator:latest --help
Error: Unable to access jarfile gtfs-validator-cli.jar

I've made the following changes in this PR to both fix the issue and add a testing step that would catch breaks in the Docker layer:

• Applied the pattern recommended by the current build-push-action for testing the Docker image before pushing
• Adjusted the CLI to return exit code 0 when invoked with --help
  • Needed a simple command to run that would return 0 if the executable is healthy
  • Other available arguments require valid input
  • It is conventional for --help to return 0 and be used to verify you can run a command, the README currently provides it as such
  • main/parseArguments have a weird split of responsibilities currently. I tried a few ways to keep System.exit() within main as it is in all other cases, but they all ended up getting more gnarly. parseArguments is already handling direct output and having side effects, so is already not a pure function.
• Fix the path in Dockerfile to the new CLI jar build output location
• Adds some more ancillary trees to .dockerignore to improve build caching when iterating locally

[bdferris-v2]

LGTM! Apologies I missed this in the previous PR. Thanks for adding a test to catch this in the future.

[isabelle-dr]

Thanks for catching this @themightychris !
Merging, thanks for providing the code review here @bdferris-v2

cc @maximearmstrong