Image Build Pipeline

Adds image building for all images in ci/images, a few things to note:

1. We'll pin the hashes of the new images built from this pipeline in a separate PR.
2. Hashes must always be pinned, since anyone with push access to this repo can push an image.
3. I added keys.openpgp.org as a GPG keyserver in the Litecoin build since the others were timing out.