build: add tag with major version (#389)

Add a tag to the built Docker image with its associated major version
number. This will allow users to get minor and patch updates for free
without having to manually change anything in their pipelines.

i.e. if the current version was `v1.2.3`, we're now adding a new `v1`
tag, along with all the previous ones (`v1.2.3`, `latest`).

closes #388

Author: sdn4z

.github/actions/docker-build/action.yml

@@ -64,6 +64,12 @@ runs:
       uses: docker/metadata-action@c1e51972afc2121e065aed6d45c65596fe445f3f # v5.8.0
       with:
         images: ${{ inputs['image-name'] }}
+        tags: |
+          type=semver,enable=true,pattern={{raw}}
+          type=semver,enable=true,pattern={{major}},prefix=v
+          type=ref,event=pr,prefix=pr-,enable=true
+          type=ref,event=branch,branch=main,pattern={{raw}}
+          type=ref,event=branch,branch=main,pattern={{major}},prefix=v
 
     - name: Build Docker image
       uses: docker/build-push-action@263435318d21b8e681c14492fe198d362a7d2c83 # v6.18.0