Docker compose Rails + Oracle DB

I am attempting to create a docker-compose file that will start a Rails 3.2.13 server and link it to an Oracle DB. So far I have been unable to make the connection from Rails to the Oracle DB. This is the error from Rails

oci8.c:513:in oci8lib_191.so: ORA-12154: TNS:could not resolve the connect identifier specified (OCIError)

docker-compose.yml

version: '3.1'

services:
    app:
        image: local-rails:0.0.1
        build: .
        ports:
            - "3000:3000"
        volumes:
            - "${PWD}:/usr/src/app"
        depends_on:
            - db
        command: bundle exec rails s -p 3000 -b '0.0.0.0'
    db:
        image: wnameless/oracle-xe-11g
        environment:
          - ORACLE_ALLOW_REMOTE=true
        ports:
            - "49160:22"
            - "49161:1521"

Variables in my Dockerfile

ENV RAILS_ENV local

# Notice the use of "db" service name as the HOST
ENV RAILS_DB (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=db)(PORT=49161))(CONNECT_DATA=(SID=xe)))

# I've also tried setting DATABASE_URL directly w/ no yml config
# ENV DATABASE_URL db:49161/xe

database.yml

local:
  adapter: oracle_enhanced
  database: <%= RAILS_DB %>
  username: ****
  password: ****

I have successfully started both services separately - Rails pointing to external Oracle DB with known working configurations. I have also been able to connect to the containerized Oracle DB via Oracle SQL Developer. The problem is getting them to talk to one another.