no pg_hba.conf entry for replication connection from host
When doing this:

$ psql -h $MASTER_PORT_5432_TCP_ADDR -U replication
psql: FATAL:  no pg_hba.conf entry for host "$SLAVE_PORT_5432_TCP_ADDR", user "replication", database "replication", SSL off


Since there's no explicit database indicated with -d (for example -d postgres ), it takes replication as the database, the same as the user name. However that's probably not what you want. replication is not supposed to be a real database, as mentioned in the doc for pg_hba.conf:

http://www.postgresql.org/docs/static/auth-pg-hba-conf.html


  The value replication specifies that the record matches if a
  replication connection is requested (note that replication connections
  do not specify any particular database). Otherwise, this is the name
  of a specific PostgreSQL database.


So if you expected that this line:

host    replication     ${REP_USER}     0.0.0.0/0               md5


would allow psql to connect with the command above, it's normal that it doesn't work, even assuming that ${REP_USER} is actually replication.

As for the other line:

host    ${DB_NAME}      ${DB_USER}      0.0.0.0/0               md5


not knowing what are the real values for ${DB_NAME} and ${DB_USER} it's not really possible to comment on it.

If the real problem is the replication not happening, I don't think psql is very useful to test this since psql must connect to a specific database before doing anything useful, whereas the replication process must not and does not.