Suppressing No pinentry warnings with GPG (in Automated Builds)

If you're trying to automagically sign things using GPG in an automated build environment, such as Jenkins or GitLab CI, you may encounter warnings, like the following, where there's No pinentry program available:

$ gpg -v --import $SIGNING_KEY
gpg: directory '/root/.gnupg' created
gpg: keybox '/root/.gnupg/pubring.kbx' created
gpg: sec  rsa3072/DF7507BC5D21FAD0 2022-01-04  Jamie Tanna <gpg-automation@jamietanna.co.uk>
gpg: /root/.gnupg/trustdb.gpg: trustdb created
gpg: using pgp trust model
gpg: key DF7507BC5D21FAD0: public key "Jamie Tanna <gpg-automation@jamietanna.co.uk>" imported
gpg: no running gpg-agent - starting '/usr/bin/gpg-agent'
gpg: waiting for the agent to come up ... (5s)
gpg: connection to agent established
gpg: key DF7507BC5D21FAD0/DF7507BC5D21FAD0: error sending to agent: No pinentry
gpg: error building skey array: No pinentry
gpg: error reading '/builds/jamietanna/cucumber-reporting-plugin.tmp/SIGNING_KEY': No pinentry
gpg: import from '/builds/jamietanna/cucumber-reporting-plugin.tmp/SIGNING_KEY' failed: No pinentry
gpg: Total number processed: 0
gpg:               imported: 1
gpg:       secret keys read: 1

A hint from this GitLab issue highlighted the fact that we can force the loopback pinentry mode, which is explained more on StackOverflow, and can be seen with the following tweak to our command:

-gpg -v --import $SIGNING_KEY
+gpg --pinentry-mode loopback -v --import $SIGNING_KEY

This then silences the warnings, so you no longer need to worry about not having a pinentry program.