Localize Alloy container image (#300)

## Summary

- Add `containers/alloy/` with dual Dockerfile + Nix build files for Grafana Alloy v1.14.0
- Both builds fetch source from forge mirror (`forge.ops.eblu.me/mirrors/alloy.git`), build the web UI (Node), then compile the Go binary with `netgo embedalloyui` tags
- Update all three alloy deployments (alloy-k8s, alloy-ringtail, alloy-tracing-ringtail) to use `registry.ops.eblu.me/blumeops/alloy`
- `promtail_journal_enabled` tag omitted — requires systemd headers and none of our configs use `loki.source.journal`

## Build verification

- **Dockerfile:** Tested locally via `docker build`, binary reports `v1.14.0` with correct tags
- **Nix:** Tested on ringtail via `nix-build`, all three hashes (fetchgit, npmDeps, goModules) resolved and build succeeds

## Post-merge steps

1. Wait for CI to build the container from main (both Dockerfile and Nix workflows)
2. `mise run container-list alloy` to find the `[main]` tagged image
3. C0 follow-up to update `newTag` in all three kustomizations from `v1.14.0-placeholder` to the real tag
4. Sync ArgoCD apps and verify pods come up healthy

Reviewed-on: #300

docs/changelog.d/feature-localize-alloy-container.infra.md

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+Localize Grafana Alloy container image with dual Dockerfile + Nix builds from forge mirror