Add Dockette CI rollout baseline

[ohmyfelix]

What changed

• Added Test, Build, and Docs jobs to the Docker workflow.
• Added baseline Makefile build, test, and run aliases while preserving existing helper targets.
• Normalized README badges to the dockette/copybara style and added the standard Maintenance section.

Why

• Aligns dockette/expose with the Dockette CI rollout baseline and keeps Docker Hub documentation updated.

[ohmyfelix]

Added CI fix commit: rebase Expose image from Buster/PHP 8.0 to Bullseye/PHP 8.2.

[f3l1x]

Suggested change: run CI on PRs, but keep publish strictly master-only

Right now this workflow has no pull_request trigger, so the Test job never validates the PR — and the publish (Build) job calls the reusable workflow with secrets: inherit with no master guard. Adding pull_request without guarding the publish job would expose registry secrets on every PR.

Two changes fix both:

 on:
   workflow_dispatch:
+
+  pull_request:

   push:
     branches: ["master"]
@@
   build:
     name: "Build"
     needs: ["test"]
+    if: github.ref == 'refs/heads/master'
     uses: dockette/.github/.github/workflows/docker.yml@master
     secrets: inherit

Result: a PR runs only the Test job (builds the image locally with load: true, runs make test, no secrets); Build (publish) and Docs run only on master. This matches the canonical shape already used in dockette/mockbin#2 and dockette/perl#2.