Fix the docker_target parameter for toolchains

[pbecotte]

We have to update the configure parameter to actually use docker_target. Once that is setup, we have to make sure that the executable is added to the runfiles, and update the entrypoint script to find the built binary. This should resolve https://github.com/bazelbuild/rules_docker/issues/2170

PR Checklist

Please check if your PR fulfills the following requirements:

• [X] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• [x] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Documentation content changes
• [ ] Other... Please describe:

What is the current behavior?

Issue Number: #2170

What is the new behavior?

Fixes the functionality to work as described

Does this PR introduce a breaking change?

• [ ] Yes
• [X] No

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