How can one access an external database from a container? Is the best way to hard code in the connection string?
# Dockerfile
ENV DATABASE_URL amazon:rds/connection?string
You can pass environment variables to your containers with the -e (alias --env) flag.
docker run -e xx=yy
An example from a startup script:
sudo docker run -d -t -i -e REDIS_NAMESPACE='staging' \
-e POSTGRES_ENV_POSTGRES_PASSWORD='foo' \
-e POSTGRES_ENV_POSTGRES_USER='bar' \
-e POSTGRES_ENV_DB_NAME='mysite_staging' \
-e POSTGRES_PORT_5432_TCP_ADDR='docker-db-1.hidden.us-east-1.rds.amazonaws.com' \
-e SITE_URL='staging.mysite.com' \
-p 80:80 \
--link redis:redis \
--name container_name dockerhub_id/image_name
Or, if you don't want to have the value on the command-line where it will be displayed by ps, etc., -e can pull in the value from the current environment if you just give it without the =:
sudo PASSWORD='foo' docker run [...] -e PASSWORD [...]
If you have many environment variables and especially if they're meant to be secret, you can use an env-file:
$ docker run --env-file ./env.list ubuntu bash
The --env-file flag takes a filename as an argument and expects each line to be in the VAR=VAL format, mimicking the argument passed to --env. Comment lines need only be prefixed with #