Add workflow to publish container images on github

Note that variable expansion in --from and FROM was working oddly,
see: https://github.com/docker/cli/issues/3356#issuecomment-957892224
for context.

python/Dockerfile

@@ -1,14 +1,17 @@
+ARG clear_ver
+FROM ghcr.io/clearlinux/os-core:$clear_ver AS core
 FROM clearlinux:latest AS builder
 
+ARG clear_ver
 ARG swupd_args
 # Move to latest Clear Linux release to ensure
 # that the swupd command line arguments are
 # correct
-RUN swupd update --no-boot-update $swupd_args
+RUN swupd update --no-boot-update -V $clear_ver $swupd_args
 
 # Grab os-release info from the minimal base image so
 # that the new content matches the exact OS version
-COPY --from=clearlinux/os-core:latest /usr/lib/os-release /
+COPY --from=core /usr/lib/os-release /
 
 # Install additional content in a target directory
 # using the os version from the minimal base
@@ -23,12 +26,12 @@ RUN source /os-release && \
 # file exists on different layers. To minimize docker
 # image size, remove the overlapped files before copy.
 RUN mkdir /os_core_install
-COPY --from=clearlinux/os-core:latest / /os_core_install/
+COPY --from=core / /os_core_install/
 RUN cd / && \
     find os_core_install | sed -e 's/os_core_install/install_root/' | xargs rm -d &> /dev/null || true
 
 
-FROM clearlinux/os-core:latest
+FROM core
 
 ENV NAME="clearlinux-python"
 COPY --from=builder /install_root /