EC2 backend docker run issues after ssh into instance - ruby-on-rails

Question

Im attempting to bring up the backend on this Ruby on Rails app using docker but am getting the following error after I try to run the following commands on my ssh'd ec2 instance.
[ec2-user#ip-10-10-3-155 ~]$ docker run --rm \
>-it --name *****-db-setup \
>839014*****.dkr.ecr.us-east-1.amazonaws.com/*****-backend:dev-latest \
>bundle exec rake db:drop db:create db:migrate
rake aborted!
PG::ConnectionBad: could not translate host name "db"
to address: Name or service not known
/usr/local/bundle/gems/pg-0.21.0/lib/pg.rb:56:in `initialize'
/usr/local/bundle/gems/pg-0.21.0/lib/pg.rb:56:in `new'
/usr/local/bundle/gems/pg-0.21.0/lib/pg.rb:56:in `connect'
/usr/local/bundle/gems/activerecord-5.1.4/lib/active_record/connection_adapters/postgresql_adapter.rb:695:in
`connect'
/usr/local/bundle/gems/activerecord-5.1.4/lib/active_record/connection_adapters/postgresql_adapter.rb:220:in
`initialize'
/usr/local/bundle/gems/activerecord-5.1.4/lib/active_record/connection_adapters/postgresql_adapter.rb:38:in `new'
/usr/local/bundle/gems/activerecord-5.1.4/lib/active_record/connection_adapters/postgresql_adapter.rb:38:in `postgresql_connection' /myapp/app/models/admin.rb:7:in `include'
/myapp/app/models/admin.rb:7:in `<class:Admin>'
/myapp/app/models/admin.rb:3:in `<top (required)>'
/usr/local/bundle/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:476:in
`load' .......Tasks: TOP => db:drop =>
db:check_protected_environments => environment
this is my env file I created
$ export AWS_ACCESS_KEY_ID=***********
$ export AWS_SECRET_ACCESS_KEY=***************************
$ export AWS_DEFAULT_REGION=us-east-1
$ export DB_USER=*****
$ export DB_PASS=****test1
$ export DB_NAME=*****_backend
$ export DB_HOST=tf-*****-backend-2021051***********.c8o12l*****.us-east-1.rds.amazonaws.com
$ export DB_PORT=5432
Docker-compose.yml
version: '3'
services:
db:
image: postgres
backend:
build: backend
# https://stackoverflow.com/a/38732187/42559
entrypoint: /myapp/entrypoint.sh
command: bundle exec unicorn -p 3001
volumes:
- ./backend/:/myapp
ports:
- "3001:3001"
depends_on:
- db
env_file:
- ./.env
frontend:
build: frontend
command: yarn server
volumes:
- ./frontend/:/app
- /app/node_modules
ports:
- "3000:3000"
depends_on:
- backend
This is the Database.yml
default: &default
adapter: postgresql
encoding: unicode
host: <%= ENV.fetch("DB_HOST") { "db" } %>
username: <%= ENV.fetch("DB_USER") { "postgres" } %>
port: <%= ENV["DB_PORT"] %>
password: <%= ENV["DB_PASS"] %>
# For details on connection pooling, see Rails configuration guide
# http://guides.rubyonrails.org/configuring.html#database-pooling
pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
development:
<<: *default
database: backend_development
test:
<<: *default
database: backend_test
production:
<<: *default
database: <%= ENV["DB_NAME"] %>
From my observations and attempts I believe it due to how the Docker-compose.yml and env file are interacting or something with my ruby server. I already have the backend image in the EC2 instance repository, now im trying to convert it to a container and bring up the backend. Thanks for any insight provided in advance !