Add proxy or magic file to purge caches

[jonasfj]

Instead of interpreting exit codes (which out abstractions deliberately don't support).

However, we could support:

• curl http://taskcluster/purge-cache/<cacheName> (from within the container)
  or
• touch /.well-known/purge-cache/<cacheName> (from within the container)

[jonasfj]

Probably using /.well-known/purge-cache/<cacheName> won't work across operating systems.