Switch CI builds to docker buildx (#146)

## Summary
- Replace deprecated `docker build` with `docker buildx build` in the build-push-image composite action
- Remove redundant build/run comments from nettest Dockerfile

## Test plan
- [ ] Merge and tag `nettest-v1.1.0` (or similar) to trigger the build-container workflow
- [ ] Verify the build succeeds without the deprecation warning

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/146

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