acbuild pod for building about build HOT 1 OPEN

Related: #86

I'm not sure of the security implications of this, since an acbuild script is normally a shell script that runs as root. If we're running it in a container that gets blown away at the end of the build, is a user uploading a script that can break out of the container a concern? If so maybe revisiting the subject of an acbuild script mode could help.

Step 1 could be accomplished with some github integration, which would be very convenient for CD reasons.

Ideally at the end of a successful build it would just push a new image to quay or a private acserver or something with acpush, which would remove the need for step 3.

Regardless of if this thing serves the image itself or pushes it somewhere, it'll need to be able to sign it for the user. I don't like the idea of asking users to upload a private key, so maybe just generate a new key on the server for each project and sign it with that? That would complicate pushing the images somewhere else though, since the current push proposal for appc has the public gpg key sent to the server OOB.

from build.