added gpg to image to support git verify-commit

[Lerentis]

hello there 👋

i tried to use this image in a CI way to check if the commit in question is verified. sadly this image does not contain a gpg package, which results in the following error:

git verify-commit $SHA
error: cannot run gpg: No such file or directory

adding gpg into the image makes it possible to use this git command:

git verify-commit $SHA
gpg: directory '/root/.gnupg' created
gpg: keybox '/root/.gnupg/pubring.kbx' created
gpg: Signature made Mon Aug 22 10:21:24 2022 UTC
gpg:                using RSA key $KEY

As this is now part of the standard git usage, i think gpg should be added to this image.

feel free to close this PR if you disagree with that 😃

[ozbillwang]

LGTM