Reconfigure Containerfile to accept FEDORA_MAJOR_VERSION as ARG input

Containerfile

@@ -1,4 +1,6 @@
-FROM ghcr.io/cgwalters/fedora-silverblue:37
+ARG FEDORA_MAJOR_VERSION=37
+
+FROM ghcr.io/cgwalters/fedora-silverblue:${FEDORA_MAJOR_VERSION}
 # See https://pagure.io/releng/issue/11047 for final location
 
 COPY etc /etc
@@ -11,4 +13,4 @@ RUN rpm-ostree override remove firefox firefox-langpacks && \
     systemctl enable rpm-ostreed-automatic.timer && \
     systemctl enable flatpak-automatic.timer && \
     rpm-ostree cleanup -m && \
-    ostree container commit
+    ostree container commit