Add robust set of dev tags for release + main branches, PRs

[nathancoleman]

Changes proposed in this PR:

For release/** branch:

• No changes, still just publish to consul-api-gateway:<version>

For main branch:

• a mutable tag, consul-api-gateway:latest that gets updated for every commit
• an immutable-ish tag, consul-api-gateway:<commit-sha> that points at each unique commit's build

For pull requests:

• a mutable tag, consul-api-gateway:pr-<number> that gets updated for every commit in the PR
• an immutable-ish tag, consul-api-gateway:<commit-sha> that points at each unique PR commit's build

How I've tested this PR:

How I expect reviewers to test this PR:

Checklist:

• Tests added
• CHANGELOG entry added

  Run make changelog-entry for guidance in authoring a changelog entry, and
  commit the resulting file, which should have a name matching your PR number.
  Entries should use imperative present tense (e.g. Add support for...)

[mikemorris]

Refs the similar implementation in Consul core:

• [CI-only] Build and publish dev dockerhub images  consul#13084
  • [CI-only] Update tagging for dev_tags consul#13199
• [release/1.11.x] Update version string on active release branches consul#13308

[nathancoleman]

Closing for now since CRT processes don't currently run for PR branches and would require some changes to do so