How to set docker env file that inside the image - docker

Question

i am a totally docker newb, so sorry for that
i have stand-alone docker image (some node app),
that i want to run in different environments.
i want to set up the env file with run RUN --env-file <path>
How ever, i want to use the env files that inside the image (so i can use different files per env),
and not on server.
so would be the path inside image.
is there any way to do so?
perhaps like "cp" (docker cp [OPTIONS] CONTAINER:<path>)
but doesn't seem to work.
what the best practice here?
am i making sense?
Thanks!!

Answer 1

Docker bind mounts are a fairly effective way to inject configuration files like this into a running container. I would not try to describe every possible configuration in your built image; instead, let that be configuration that's pushed in from the host.
Pick some single specific file to hold the configuration. For the sake of argument, let's say it's /usr/src/app/env. Set up your application however it's built to read that file at startup time. Either make sure the application can still start up if the file is missing, or build your image with some file there with reasonable default settings.
Now when you run your container, it will always read settings from that known file; but, you can specify a host file that will be there:
docker run -v $PWD/env.development:/usr/src/app/env myimage
Now you can locally have an env.development that specifies extended logging and a local database, and an env.production with minimal logging and pointing at your production database. If you set up a third environment (say a shared test database with some known data in it) you can just run the container with this new configuration, without rebuilding it.

Answer 2

Following is the command to run docker
docker run [OPTIONS] IMAGE [COMMAND] [ARG...]
Example
docker run --name test -it debian
focus on following switch
--env , -e Set environment variables
--env-file You can pass environment variables to your containers with the -e flag.
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
In case, 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 #