Post image CD change cleanup

[rainest]

Properly tracking tasks to complete after #57.

Once that's in place, we need to do post-merge evaluation of the new CD workflow. For basic functionality:

• Confirm PR preview images work as expected.
• Confirm tip of main preview images work as expected.
• Confirm tag images and releases work as expected.

I expect these to be fine based on prior testing, but the best laid plans of mice and men oft do go awry and all.

For leftover cleanup and incomplete features once we've done basic vetting:

• Remove the GoReleaser workflow.
• Rename Dockerfile.build to Dockerfile.
• Add label filters to PR preview builds, to not waste Actions run time/GCHR image tags on things we'll never use/that are identical to tip of main builds.
• Target another repo for conversion once we're comfortable power-control has all kinks worked out.