Switch CI builds from docker build to docker buildx build

The legacy Docker builder is deprecated and will be removed in a future
release. BuildKit (via buildx) is the replacement.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.forgejo/actions/build-push-image/action.yaml

@@ -30,7 +30,7 @@ runs:
       shell: bash
       run: |
         echo "Building ${{ inputs.image_name }}:${{ inputs.version }}"
-        docker build \
+        docker buildx build \
           --tag ${{ inputs.image_name }}:${{ inputs.version }} \
           --file ${{ inputs.context }}/${{ inputs.dockerfile }} \
           ${{ inputs.context }}

containers/nettest/Dockerfile

@@ -3,12 +3,6 @@
 # This container tests connectivity to tailnet services from various environments:
 # - Docker on indri (during CI build)
 # - Minikube pods (manual testing)
-#
-# Build:
-#   docker build -t registry.ops.eblu.me/blumeops/nettest:latest .
-#
-# Run:
-#   docker run --rm registry.ops.eblu.me/blumeops/nettest:latest
 
 FROM alpine:3.21
 

docs/changelog.d/fix-docker-buildx.infra.md

@@ -0,0 +1 @@
+Switch CI container builds from deprecated `docker build` to `docker buildx build` (BuildKit).