fix(docker): Add edge tag to Docker images, document when latest tag will appear (#5312)

* Add latest and edge tags to Docker images

* Document how latest tag actually works

* Try a different syntax for is_default_branch

* Try again

* One last try

* Revert changes that don't work

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.github/workflows/build-docker-image.yml

@@ -75,12 +75,17 @@ jobs:
           # generate Docker tags based on the following events/attributes
           tags: |
             type=schedule
+            # semver and ref,tag automatically add a "latest" tag, but only on stable releases
             type=semver,pattern={{version}}
             type=semver,pattern={{major}}.{{minor}}
             type=semver,pattern={{major}}
+            type=ref,event=tag
             type=ref,event=branch
             type=ref,event=pr
             type=sha
+            # edge is the latest commit on the default branch.
+            # TODO: We only want edge builds for `us-docker.pkg.dev`, because DockerHub doesn't keep up with every `main` branch commit.
+            type=edge,enable={{is_default_branch}}
 
       # Setup Docker Buildx to allow use of docker cache layers from GH
       - name: Set up Docker Buildx