Use environment variables in CMD
This answer may be a little late. But environment for CMD is interpreted slightly differently depending on how you write the arguments. If you pass the CMD as a string (not inside an array), it gets launched as a shell instead of exec. See https://docs.docker.com/engine/reference/builder/#cmd.
You may try the CMD without the array syntax to run as a shell:
CMD myserver --arg=$ARG --memcache=$MEMCACHE_11211_TCP_ADDR:$MEMCACHE_11211_TCP_PORT
Both Andys had it right. The json syntax bypasses the entrypoint. When you use CMD as in their example, it is considered as an argument to the default entrypoint: /bin/sh -c which will interpret the environement variables.
Docker does not evaluate the variables in CMD in either case. In the former, the command is directly called so nothing gets interpreted, in the later, the variables are interpreted by sh.