Use correct source for container image

[lkiesow]

The workflow for building the test container images is triggered by pull_request_target which means that we explicitly need to state that we want to check out the pull request source code and not what is already in the repository.

[github-actions]

Use docker or podman to test this pull request locally.

Local test with mock data

podman run --rm -it -p 127.0.0.1:3000:3000 ghcr.io/opencast/opencast-admin-interface:pr-492

Proxy data from develop.opencast.org

podman run --rm -it -p 127.0.0.1:3000:3000 -e PROXY_TARGET=https://develop.opencast.org ghcr.io/opencast/opencast-admin-interface:pr-492

It may take a few seconds for the interface to spin up.
It will then be available at http://127.0.0.1:3000.
For more options you can pass on to the proxy, take a look at the README.md.

[lkiesow]

Thanks for reviewing this, @Arnei. I'm merging this since this may cause people to test incorrect code otherwise. That could have bad side effects especially when the BugBash starts on Tuesday.