build image: use system TS instead of commit TS
build image: use system TS instead of commit TS

The build image build install the _latest_ version of several
dependencies. Hence the actual contents depend on the actual time of
the build more than the time of the commit. So differentiate on both
commit hash and build timestamp.
treasure-data/digdag,treasure-data/digdag,treasure-data/digdag,treasure-data/digdag,treasure-data/digdag,treasure-data/digdag,treasure-data/digdag