Install gpg2 and forward gpg-agent

For signing git commits, git uses GnuPG as an external program, defaulting to `gpg`. That binary is provided in the container, the problem with it is, it cannot use the gpg-agent as provided by Seahorse etc. and bails out on needing to supply a passphrase without a terminal.

The gpg program name can be overridden to `gpg2` in git configuration, but this binary is not present in the container. Neither, I believe, the agent socket is forwarded from the desktop.