[ci][dcoker] Make Fedora version controlled by env variable

Similarly to the Ubuntu container, this will help in building multiple Fedora version from the same docker file. It will be used to add support for Fedora 36.

docker/build/Dockerfile.fedora

@@ -1,7 +1,8 @@
 # Copyright (c) PLUMgrid, Inc.
 # Licensed under the Apache License, Version 2.0 (the "License")
 
-FROM fedora:34
+ARG VERSION="34"
+FROM fedora:${VERSION}
 
 ARG RUBY_INSTALL_VERSION="0.8.4"
 ENV RUBY_INSTALL_VERSION=$RUBY_INSTALL_VERSION