For pinning a docker image to a project please check this post:
As for the additional commits, after they are pushed you have the option to start a new environment from the new commit (after the image has built successfully or immediately if its pinned).
You can always choose which commit/branch you want to start your environment from (see screenshot below, drop-down menu), but you can only start one environment per commit. It is also possible to pull the latest commits in an environment that started with an old commit.
