dmaketgz: accept a SOURCE_DATE_EPOCH as an second argument

to make it easier to reproduce a tarball

Closes #13573

dmaketgz: compacter

Removes the need for disabling shellcheck warnings.

Follow-up to d28f74913c2
Proposed-by: Viktor Szakats
Closes #13391

dmaketgz: release tarball generation using docker

For easier reproducibility.

Mention using this script in RELEASE-PROCEDURE

Closes #13388