enhance: bind-mount job paths through the docker proxy (#1226)

Containers a job starts through its Docker socket, for example `docker run -v "$PWD:/src"`, `./data:/data` in docker compose, or actions like dockerfile-roast, can now bind-mount the workspace and other paths the job sees, as on a host, without `bind_workdir`.

The per-job Docker proxy rewrites container and volume create requests. A bind source, or the device of a `local` volume with `o: bind`, that lies under one of the job container's mounts is pointed at that mount's path on the daemon, read from inspecting the job container. Paths that already name a daemon path, like `GITEA_DOCKER_WORKSPACE`, and paths outside the job's mounts pass through unchanged.

The proxy now also starts when the runner runs in a container given the host's Docker socket, by placing its socket in the runner's working directory, and in rootless dind, by granting the daemon socket's group through an ACL.

Fixes https://gitea.com/gitea/runner/issues/1219
Fixes https://gitea.com/gitea/runner/issues/1193

Reviewed-on: https://gitea.com/gitea/runner/pulls/1226
Reviewed-by: bircni <[email protected]>
Co-authored-by: silverwind <[email protected]>

README.md

@@ -268,7 +268,7 @@ Unlike GitHub, a job whose steps run on the host (a `host` label without `contai
 
 #### Docker from a job (`GITEA_DOCKER_WORKSPACE`)
 
-A container a job starts through the Docker socket cannot bind-mount the workspace by the job's own path, the daemon does not have it. `GITEA_DOCKER_WORKSPACE` holds the path the daemon sees. Use it as the prefix of workspace binds, with `.` as the fallback for local use, here in a `docker-compose.yaml`:
+Containers a job starts through the Docker socket can bind-mount the workspace by the job's own path, as on a host, for example `docker run -v "$PWD:/src"`. Without the Docker proxy, for example with a remote `DOCKER_HOST`, use `GITEA_DOCKER_WORKSPACE`, the path the daemon sees, as the prefix of workspace binds, with `.` as the fallback for local use:
 
 ```yaml
 volumes: