Add GitLab CI support to enable migration to GitLab

[ghost]

This file is a configuration file for GitLab's CI system, which should help microG migrate to GitLab (if desired)

There are several environment variables inside the CI that should remain private; microG will need to fill those in within GitLab's protected variables setting:

• $KEYSTORE: base64 encoded Java keystore

• $KEYSTORE_PASSWORD: the password for the keystore

• $KEY_PASSWORD: the password for the key stored in the keystore

• ${KEYSTORE_ALIAS}: the keystore alias that should be used

[ILoveOpenSourceApplications]

@ale5000-git, depending on whether this PR is needed or not, shouldn't an action be taken here?