Bind mount /mnt for temporary build artifacts

[agners]

The new GitHub Action hosted runners have less space on /. For some add-ons this can be problematic. Unfortunately it is not straight forward to use /mnt as Docker data root. Instead just mount /mnt inside the container so add-on Dockerfiles can use that location for temporary build files.

[agners]

I've realized this approach has a major drawback: To use that additional space we have to adjust the add-on Dockerfiles to use /mnt which essentially means we add GH action specific logic in Dockerfiles. This is problematic when running test builds of an add-on locally. I'd have to bind mount something to /mnt as well 😰

We could of course bind mount just a temporary volume there, and maybe even add that to the builder. But it is all a bit wonky. I think we need a better solution here 🤔

[agners]

This is too hacky. Just make sure Dockerfiles don't use as much space 😅 . Closing.