Implement non-default signal handler for SIGINT

Pressing Ctrl-C will stop a build, but does not clean up the /tmp
directory of dockerfiles. Implement a signal handler such that clean up
does occur.

Python apparently ignores signals while calling join on a queue, so
that's why the small modification was needed there.

Change-Id: I43faf67726c154b6640d3596aea61ffc59871999
Closes-Bug: #1579242