use semantic version tagging for docker images

we've never tagged stable releases, but should really do so.  This
change will cause our docker images to be tagged following typical
semver fashion... the initial `v1.0.0` release will be tagged `v1`,
`v1.0`, `v1.0.0`, and `latest`.

Updates #104

Signed-off-by: Will Norris <will@tailscale.com>

.github/workflows/docker.yml

@@ -45,6 +45,11 @@ jobs:
         uses: docker/metadata-action@96383f45573cb7f253c731d3b3ab81c87ef81934 # v5.0.0
         with:
           images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=ref,event=branch
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern={{major}}
 
       - name: Build and push Docker image
         id: build-and-push