Merge remote-tracking branch 'remotes/spesmilo/appimage-debian-base'

Change Docker base images from Ubuntu to Debian, and use `snapshot.debian.org` as apt source list.
Ubuntu occasionally removes version-pinned packages from apt (see #7484), which
- breaks historical reproducible builds
- introduces maintenance burden as we have to update the version pins
Hopefully this change fixes both issues.

merges https://github.com/spesmilo/electrum/pull/7926
closes https://github.com/spesmilo/electrum/issues/7484

.cirrus.yml

@@ -168,6 +168,7 @@ task:
     path: "contrib/build-wine/dist/*"
   env:
     CIRRUS_WORKING_DIR: /opt/wine64/drive_c/electrum
+    CIRRUS_DOCKER_CONTEXT: contrib/build-wine
 
 task:
   name: Android build
@@ -206,6 +207,8 @@ task:
     - ./contrib/build-linux/appimage/make_appimage.sh
   binaries_artifacts:
     path: "dist/*"
+  env:
+    CIRRUS_DOCKER_CONTEXT: contrib/build-linux/appimage
 
 task:
   container: