enable image configuration

[davidblum]

Overview

This PR adds optional configuration for the base shellcheck image.

Rationale

Our organization prefers to store docker images in an internal registry, by enabling this configuration
option, we can continue to use the plugin with our specified registry

[mcncl]

@davidblum thanks for the PR! I've added some tests for your additional config, hope that's OK!

[davidblum]

@mcncl Awesome!

Thank you very much for adding those!

It looks like the building is failing, I'll see if I can figure anything out.

[jeremybumsted]

@davidblum looks like the build is failing because the selftest step is trying to check out the commit which only exists on your fork. Let me see if I can get it sorted so we can get a green build, but I think as long as the other tests pass I'm happy for us to merge this so we aren't blocking you.

[davidblum]

@jeremybumsted ah, that makes sense.

I'm ok with that plan as long as it work for you and the team!

[davidblum]

@jeremybumsted I wasn't able to make any progress on my end.

Have you made any progress?

[jeremybumsted]

@davidblum I think I've got a solution, see this commit.

And looks like we got it: https://buildkite.com/buildkite/plugins-shellcheck/builds/62#01924e97-b137-4009-927e-a6dba0f24fd5/3-4

I also added another test just to try and cover all our bases on this change 😄

[jeremybumsted]

This looks good - both @mcncl and myself have had a look through this and added some tests to cover some scenarios for setting that custom image - thanks for raising this pr @davidblum 🙌

[davidblum]

Woohoo!

Thank you both so much!