Add the capability to cancel the build by disconnecting the client.
This adds a `cancelled` channel which is used to signal that a build
should halt. The build is halted by sending a Kill signal and noticing
that the cancellation channel is closed.
This first pass implementation does not allow cancellation during a
pull, but that will come in a subsequent PR.
* Add documentation of cancellation to cli and API
* Protect job cancellation with sync.Once
* Add TestBuildCancelationKillsSleep
* Add test case for build cancellation of RUN statements.
Signed-off-by: Peter Waller <p@pwaller.net>
e9942aeb8a
