Unify container build workflows (#306)

## Summary
- Merges `build-container.yaml` and `build-container-nix.yaml` into a single workflow
- Detect job classifies each changed container by presence of `Dockerfile` and/or `default.nix`
- Dockerfile containers build on `k8s` (indri) via Dagger; Nix containers build on `nix-container-builder` (ringtail) via nix-build + skopeo
- Containers with both build files (alloy, nettest, ntfy) get built on both runners

## Test plan
- [ ] Push a change to a Dockerfile-only container (e.g. grafana) — verify it builds on k8s only
- [ ] Push a change to a nix-only container (e.g. jobsync) — verify it builds on nix-container-builder only
- [ ] Push a change to a dual container (e.g. ntfy) — verify it builds on both runners
- [ ] Test workflow_dispatch with a specific container name

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #306

docs/how-to/deployment/build-container-image.md

@@ -117,7 +117,7 @@ Existing containers demonstrate several build approaches:
 | Multi-stage with Node + Go | [[#navidrome]] | Separate UI and backend build stages |
 | Multi-stage Elixir | [[#teslamate]] | Elixir release with Node assets |
 | Runtime tarball download | [[#kiwix-serve]] | Download pre-built binary with arch detection |
-| Nix `dockerTools` | [[#nettest-nix]] | `buildLayeredImage` with nixpkgs tools |
+| Nix `dockerTools` | [[#ntfy-nix]] | `buildLayeredImage` with nix-built app |
 
 ### transmission
 
@@ -139,9 +139,9 @@ Existing containers demonstrate several build approaches:
 
 `containers/kiwix-serve/Dockerfile` — Downloads a pre-built binary from upstream, with architecture detection for cross-platform support.
 
-### nettest (nix)
+### ntfy (nix)
 
-`containers/nettest/default.nix` — Uses `dockerTools.buildLayeredImage` with `buildEnv` to merge nixpkgs tools (curl, jq, dnsutils, bash). Runs alongside the existing Dockerfile; the nix variant is tagged `:version-nix` in the registry.
+`containers/ntfy/default.nix` — Builds ntfy from source using `buildGoModule` and packages it with `dockerTools.buildLayeredImage`. Runs alongside the existing Dockerfile; the nix variant is tagged `:version-nix` in the registry.
 
 ## Related