Use Docker Buildx for multi-arch builds

The bitwarden_rs code is still cross-compiled exactly as before, but Docker
Buildx is used to rewrite the resulting Docker images with correct platform
metadata (reflecting the target platform instead of the build platform).
Buildx also now handles building and pushing the multi-arch manifest lists.

hooks/arches.sh

@@ -1,19 +1,16 @@
-# The default Debian-based images support these arches for all database connections
-#
-# Other images (Alpine-based) currently
-# support only a subset of these.
+# The default Debian-based images support these arches for all database backends.
 arches=(
     amd64
-    arm32v6
-    arm32v7
-    arm64v8
+    armv6
+    armv7
+    arm64
 )
 
 if [[ "${DOCKER_TAG}" == *alpine ]]; then
-    # The Alpine build currently only works for amd64.
-    os_suffix=.alpine
+    # The Alpine image build currently only works for certain arches.
+    distro_suffix=.alpine
     arches=(
         amd64
-        arm32v7
+        armv7
     )
 fi