Add `examples/docker/opencode.Dockerfile` example

[sergeyklay]

Add a Docker example image examples/docker/opencode.Dockerfile that packages Sortie and the OpenCode CLI in a single container. Mirrors the existing claude-code.Dockerfile, codex.Dockerfile, and copilot.Dockerfile.

The image should include:

• Base: the same base image used by the existing agent Dockerfiles.
• Sortie binary installation (release artifact, matching architecture).
• OpenCode CLI installation (npm package or binary release, per the distribution channel documented in Research OpenCode CLI: flags, subprocess behavior, output format, session lifecycle #475).
• Any runtime dependencies OpenCode requires (Node.js, git, shell tooling).
• Entrypoint consistent with the other example Dockerfiles.

Update examples/docker/README.md to list the new image alongside the existing entries.

Blocked by #475.

Verify: docker build -f examples/docker/opencode.Dockerfile . produces a working image. Running the image with a minimal WORKFLOW.opencode.md mounted completes a turn against a dummy issue.