The build arg can be used to either test different versions, but
also makes it easier to "grep" when upgrading versions.
The output format of `gpg2 --list-secret-keys` changed in the version
installed on Buster, so `grep` was replaced with `awk` to address
the new output format;
Debian Jessie:
gpg2 --no-auto-check-trustdb --list-secret-keys
/root/.gnupg/secring.gpg
------------------------
sec 1024D/A7B21401 2018-04-25
uid Sakuya Izayoi <sakuya@gensokyo.jp>
ssb 1024g/C235E4CE 2018-04-25
Debian Buster:
gpg2 --no-auto-check-trustdb --list-secret-keys
/root/.gnupg/pubring.kbx
------------------------
sec dsa1024 2018-04-25 [SCA]
9781B87DAB042E6FD51388A5464ED987A7B21401
uid [ultimate] Sakuya Izayoi <sakuya@gensokyo.jp>
ssb elg1024 2018-04-25 [E]
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
aiordache