docker: Add --from-git build option

* docker/docker-build.sh (--from-git|--git|-g): Add option
to update frontend packages from git.

This can be useful to setup CI or just to get a testing
build of the current development state.

For now this updates:

gpgme
libkleo
kleopatra
gpgol
gpgoljs
gpgpass
gpg4win-tools
mimetreeparser

To update additional pacakges see packages/gen-tarball.sh.