CODE EXAMPLE FOR SHELL

docker redis default username password

$ docker run --name some-redis -d redis redis-server --requirepass foobared

# verify it worked with
$ docker run -it --link some-redis:redis --rm redis redis-cli -h redis -p 6379
redis:6379> AUTH test
(error) ERR invalid password
redis:6379> AUTH foobared
OK
redis:6379> 


This could be abstracted to an environment variable trivially via something like:
$ docker run --name some-redis -d -e REDIS_PASSWORD=foobared redis sh -c 'exec redis-server --requirepass "$REDIS_PASSWORD"'