Allow building Docker image via script

[fapdash]

• callers can pass in the optional variable docker_build_script
• script should produce a Docker image
• if the script variable is set the default Docker image build step isn't executed

See exercism/java-test-runner#147 for context

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Would be nice to have caching, not sure how hard it is to add.

[fapdash]

Is there a recommended way to test this?

[ErikSchierboom]

I have to think this through a bit, and that takes a bit more time with me no longer being employed at Exercism. There are a couple of things that spring to mind:

• Caching (as you mentioned)
• Security (execution of a script, probably fine due to test runner PRs requiring approval)
• Ensure that when the script executes successfully, that a Docker image has actually been built
• Ensure that when the script executes successfully, the Docker image has the right name
• Returning a correct error code

So I think we'd at least need to have a wrapper script to handle all this. That is, if @iHiD feels like this is worth the effort (and he's very busy right now).