feat(P03): docker release — multi-stage Dockerfile + Gitea container registry publish

REQ-046: Docker image published to Gitea container registry per release.

Dockerfile: multi-stage (golang:1.25 -> distroless/static-debian12:nonroot).
  CGO_ENABLED=0, ORCA_HOME=/var/lib/orca, ENTRYPOINT [/orca].
  Image size: ~28MB. Runs as nonroot.

.coreci.yml: new container-publish step in release pipeline (docker:24-cli,
  builds + tags + login + push + logout).

scripts/release.sh: docker build + push after Gitea release. Graceful
  skip if docker absent or GITEA_TOKEN unset. Env-overridable registry.

.dockerignore: excludes .git, bin/, .env, .ciagent/, testdata/, *.tar.gz.

docs/docker.md: pull, run, state persistence (volume mount), local build,
  manual publish guide.

Verified: docker build + run version/init with volume persistence.

---ci---
project: orca
phase: 3
milestone: v0.5
status: verify
---/ci---

docs/docker.md

@@ -0,0 +1,96 @@
+# Docker Guide
+
+Orca is available as a container image on the Gitea container registry.
+The image is a minimal distroless static build (~2MB runtime layer)
+that runs the orca binary directly.
+
+## Image
+
+```
+git.cloudinit.dev/coreci/orca:<version>
+git.cloudinit.dev/coreci/orca:latest
+```
+
+The image is built from the `Dockerfile` in the repo root:
+- **Build stage**: `golang:1.25` — compiles a static binary with
+  `CGO_ENABLED=0` (modernc/sqlite is pure Go, no CGO).
+- **Runtime stage**: `gcr.io/distroless/static-debian12:nonroot` —
+  ~2MB, no shell, runs as `nonroot` user.
+
+## Pull
+
+```bash
+docker pull git.cloudinit.dev/coreci/orca:latest
+# or pin a version
+docker pull git.cloudinit.dev/coreci/orca:v0.4.4
+```
+
+The repo is public (REQ-045), so anonymous pull works without login.
+
+## Run
+
+```bash
+# Print version
+docker run --rm git.cloudinit.dev/coreci/orca:v0.4.4 version
+
+# Initialize state (creates /var/lib/orca/ inside the container)
+docker run --rm -v orca-data:/var/lib/orca git.cloudinit.dev/coreci/orca:v0.4.4 init
+
+# Run the daemon (persist state via volume)
+docker run -d --name orca \
+  -p 8080:8080 \
+  -v orca-data:/var/lib/orca \
+  git.cloudinit.dev/coreci/orca:v0.4.4 daemon --addr=:8080
+```
+
+## State Persistence
+
+The image sets `ENV ORCA_HOME=/var/lib/orca`. All orca state (SQLite
+database, CA certs, server certs) is written under this path. To
+persist state across container restarts, mount a volume:
+
+```bash
+docker volume create orca-data
+docker run --rm -v orca-data:/var/lib/orca git.cloudinit.dev/coreci/orca:v0.4.4 init
+docker run -d --name orca -p 8080:8080 -v orca-data:/var/lib/orca git.cloudinit.dev/coreci/orca:v0.4.4 daemon
+```
+
+Without a volume, state is lost when the container exits.
+
+## System-Level Namespace Inside Containers
+
+The `--system` flag is not needed inside containers — the image already
+sets `ORCA_HOME=/var/lib/orca`. Use `--system` only if you want a
+different namespace root (e.g., `/root/.orca`), which requires running
+as root (the distroless image runs as `nonroot` by default).
+
+## Build Locally
+
+```bash
+docker build --build-arg VERSION=v0.4.4 -t orca-local:v0.4.4 .
+docker run --rm orca-local:v0.4.4 version
+```
+
+Build args:
+- `VERSION` — semver tag (injected via `-ldflags`)
+- `GIT_COMMIT` — short commit hash
+- `BUILD_TIME` — ISO 8601 build timestamp
+
+## Publish (for maintainers)
+
+The `.coreci.yml` release pipeline includes a `container-publish` step
+that builds and pushes the image on every tag release. To publish
+manually:
+
+```bash
+export GITEA_TOKEN=<token>
+docker build --build-arg VERSION=v0.4.4 -t git.cloudinit.dev/coreci/orca:v0.4.4 -t git.cloudinit.dev/coreci/orca:latest .
+echo "$GITEA_TOKEN" | docker login git.cloudinit.dev -u cloudinit-bot --password-stdin
+docker push git.cloudinit.dev/coreci/orca:v0.4.4
+docker push git.cloudinit.dev/coreci/orca:latest
+```
+
+## See Also
+
+- [Install Guide](install.md) — binary install (alternative to Docker).
+- [Namespace and Paths](namespace.md) — `ORCA_HOME` and `--system` flag.