Make `bst-here` more flexible

Having used bst-here for a little while now I find that while it works quite well, it lacks some flexibility in terms of the system running in the container.

It would be nice if:

• We could support changing the image used for the container. We should still only support buildstream/buildstream-fedora upstream, but users may write plugins that require different host tools. Such users might need to use a different image to use bst-here, and currently need to hand-edit the file.

I propose adding both a -i CLI flag as well as a BST_HERE_DOCKER_IMAGE environment variable, so that the user can set a default, but there's still a way to change it temporarily.

• We could support adding additional volumes. This might be nice to, for example, include keys or test environments into the container.

This would probably work best if we mimic the docker -p flag, since we are already trying to be somewhat consistent with the docker CLI.