enhance: add runner.tool_cache_mode and default it to none (#1171)

The current shared tools cache is not concurrency-safe, e.g. multiple jobs can write and corrupt it, for example `setup-go` with explicit go version under concurrency reliably corrupts the tool cache and fails all jobs.

This adds a new `runner.tool_cache_mode` (and `--tool-cache-mode` exec option) option which defaults to unshared tools cache:

- `none` mounts nothing, so a job uses what its image ships there and discards what it installs
- `shared` keeps the single volume every job reuses, and warns when `runner.capacity` is above 1

Under `none` effective tool cache can only come from the image or host, which is the same as it is on GitHub Actions which ships many preinstalled tools in its fat VM images.

Co-authored-by: bircni <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1171
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

internal/pkg/config/config.example.yaml

@@ -93,6 +93,12 @@ runner:
   # terminal; tools like `docker build` emit redrawing progress frames into the captured log
   # when a TTY is present.
   #allocate_pty: false
+  # What to mount at RUNNER_TOOL_CACHE (/opt/hostedtoolcache), where setup actions install tools:
+  #   none:   nothing. A docker job sees what its image ships there, a host job an empty dir, and
+  #           either way what it installs is gone when the job ends.
+  #   shared: one tool cache every job reuses. Two jobs writing the same tool version at once
+  #           corrupt it, so use it only with capacity 1.
+  #tool_cache_mode: none
   # Optional executable on the host, run once after each task's built-in cleanup
   # (post-steps, container teardown, bind-workdir removal). Additive only.
   #
@@ -195,7 +201,7 @@ container:
   #privileged: false
   # Any other options to be used when the container is started, for example:
   #   options: --add-host=my.gitea.url:host-gateway
-  # A volume declared here replaces the one the runner mounts on the same container path, so the
+  # A volume declared here replaces the one the runner would mount on the same container path, so the
   # tool cache can be kept on the host. Its source must also be allowed by valid_volumes below:
   #   options: --volume /host/toolcache:/opt/hostedtoolcache
   #options: