fix: allow relocating the tool cache and mounting over runner paths (#1122)

1. Docker rejects two mounts on one target, so a `container.volumes:` or `--volume` aimed at `/opt/hostedtoolcache` failed the job with `Duplicate mount point`. Job and service volumes now displace the mount on the same path, and `name:/target:ro` no longer mounts read-write at the literal path `/target:ro`.
1. Setting `RUNNER_TOOL_CACHE` only changed what the variable said, the cache stayed where it was, so tools writing to it landed outside the mount and `${{ runner.tool_cache }}` disagreed with the variable. It now relocates the cache. Leaving it unset behaves as before.
1. Unknown `config.yaml` keys now warn instead of being dropped without a trace.

Fixes https://gitea.com/gitea/runner/issues/813

---------

Co-authored-by: bircni <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1122
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

act/container/linux_container_environment_extensions.go

@@ -66,12 +66,15 @@ func (*LinuxContainerEnvironmentExtensions) JoinPathVariable(paths ...string) st
 	return strings.Join(paths, ":")
 }
 
+// DefaultToolCache is where the runner mounts the tool cache inside job containers.
+const DefaultToolCache = "/opt/hostedtoolcache"
+
 func (*LinuxContainerEnvironmentExtensions) GetRunnerContext(ctx context.Context) map[string]any {
 	return map[string]any{
 		"os":         "Linux",
 		"arch":       RunnerArch(ctx),
 		"temp":       "/tmp",
-		"tool_cache": "/opt/hostedtoolcache",
+		"tool_cache": DefaultToolCache,
 	}
 }