[LXC] global lock on start

Since the start script may create LXC templates that are shared, they
may race against each other when running for the first time. A lock
global to the host needs to be used to guarantee that does not happen.

pkg/runner/run_context.go

@@ -194,6 +194,8 @@ var lxcHelpers string
 
 var startTemplate = template.Must(template.New("start").Parse(`#!/bin/bash -e
 
+exec 5<>/tmp/forgejo-runner-lxc.lock ; flock --timeout 21600 5
+
 LXC_CONTAINER_CONFIG="{{.Config}}"
 LXC_CONTAINER_RELEASE="{{.Release}}"