rebuild binaries on demand only

Once the CTF started, you want to be able to make small fixes in the docker image without rebuilding the challenge binary since rebuilding it means you will need to distribute the new binary to the players.

This doesn't work with the current proposed way to build challenge binaries (as part of the dockerimage) since redeploying will result in a new binary.

We should have a way to
* deploy with / without rebuilding the binary
* get the build outputs

During challenge development, the default should be build=true (otherwise you forget about it and are surprised). During the competition the default should be build=false (otherwise you rebuild by mistake and might not notice).