Avoid variable expansion in --from and FROM being odd

docker/cli#3356 (comment)

nginx/Dockerfile

@@ -1,7 +1,8 @@
 ARG clear_ver
+FROM ghcr.io/bryteise/os-core:$clear_ver AS core
 FROM clearlinux:latest AS builder
-ARG clear_ver
 
+ARG clear_ver
 ARG swupd_args
 # Move to latest Clear Linux release to ensure
 # that the swupd command line arguments are
@@ -10,8 +11,7 @@ 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=ghcr.io/bryteise/os-core:$clear_ver /usr/lib/os-release /
-ARG clear_ver
+COPY --from=core /usr/lib/os-release /
 
 # Install additional content in a target directory
 # using the os version from the minimal base
@@ -26,13 +26,11 @@ 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
-ARG clear_ver
-COPY --from=ghcr.io/bryteise/os-core:$clear_ver / /os_core_install/
-ARG clear_ver
+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 ghcr.io/bryteise/os-core:$clear_ver
+FROM core
 
 COPY --from=builder /install_root /
 COPY default.conf /etc/nginx-mainline/conf.d/default.conf