Build Loki container image locally (#280)

## Summary
- Add two-stage Dockerfile for Loki (Go build → Alpine runtime) in `containers/loki/`
- Rewrite kustomize image to `registry.ops.eblu.me/blumeops/loki`
- Tag is `v3.6.5-placeholder` until first CI build; will be updated post-build

## Details
- UID 10001 matches existing StatefulSet `securityContext` (runAsUser/fsGroup)
- CGO_ENABLED=0, ldflags embed version via `github.com/grafana/loki/v3/pkg/util/build`
- Clones from `forge.ops.eblu.me/mirrors/loki` (mirror created this session)
- Pattern follows miniflux (two-stage Go) + prometheus (ldflags)

## Deployment and Testing
- [ ] Trigger container build: `mise run container-build-and-release loki`
- [ ] Update kustomize tag to actual build tag
- [ ] Deploy from branch: `argocd app set loki --revision feature/loki-container && argocd app sync loki`
- [ ] Verify `/ready` endpoint and log ingestion
- [ ] After merge: update to `[main]` tag (C0 follow-up)

Reviewed-on: #280

docs/changelog.d/feature-loki-container.infra.md

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+Build Loki container image locally instead of pulling from upstream