Dockerfile.rhel: fix scl invocation
`scl` was interpreting everything up to the last arg as other SCLs to
enable. We can explicitly tell it to stop considering args as SCLs with
`--`.
@@ -25,4 +25,4 @@ RUN chmod g=u /etc/passwd
# run as `builder` user
USER builder
-ENTRYPOINT ["/usr/bin/dumb-init", "scl", "enable", "rh-python36", "/usr/bin/coreos-assembler"]
+ENTRYPOINT ["/usr/bin/dumb-init", "scl", "enable", "rh-python36", "--", "/usr/bin/coreos-assembler"]
