Build and tag Dangerzone images

Build Dangerzone images and tag them with a unique ID that stems from
the Git reop. Note that using tags as image IDs instead of regular image
IDs breaks the current Dangerzone expectations, but this will be
addressed in subsequent commits.

install/common/build-image.py

@@ -59,7 +59,7 @@ def main():
     dirty_ident = secrets.token_hex(2)
     tag = (
         subprocess.check_output(
-            ["git", "describe", "--first-parent", f"--dirty=-{dirty_ident}"],
+            ["git", "describe", "--long", "--first-parent", f"--dirty=-{dirty_ident}"],
         )
         .decode()
         .strip()[1:]  # remove the "v" prefix of the tag.