Passing variable from container start to file

Question

I have the following lines in a Dockerfile where I want to set a value in a config file to a default value before the application starts up at the end and provide optionally setting it using the -e option when starting the container.

I am trying to do this using Docker's ENV commando

 ENV CONFIG_VALUE default_value
 RUN sed -i 's/CONFIG_VALUE/'"$CONFIG_VALUE"'/g' CONFIG_FILE
 CMD command_to_start_app

I have the string CONFIG_VALUE explicitly in the file CONFIG_FILE and the default value from the Dockerfile gets correctly substituted. However, when I run the container with the added -e CONFIG_VALUE=100 the substitution is not carried out, the default value set in the Dockerfile is kept.

When I do

docker exec -i -t container_name bash

and echo $CONFIG_VALUE inside the container the environment variable does contain the desired value 100.

Answer 1

That shouldn't be possible in a Dockerfile: those instructions are static, for making an image.

If you need runtime instruction when launching a container, you should code them in a script called by the CMD directive.
In other words, the sed would take place in a script that the CMD called. When doing the docker run, that script would have access to the environment variable set just before said docker run.

Answer 2

Instructions in the Dockerfile are evaluated line-by-line when you do docker build and are not re-evaluated at run-time.

You can still do this however by using an entrypoint script, which will be evaluated at run-time after any environment variables have been set.

For example, you can define the following entrypoint.sh script:

#!/bin/bash

sed -i 's/CONFIG_VALUE/'"$CONFIG_VALUE"'/g' CONFIG_FILE
exec "$@"

The exec "$@" will execute any CMD or command that is set.

Add it to the Dockerfile e.g:

COPY entrypoint.sh /
RUN chmod +x /entrypoint.sh
ENTRYPOINT ["/entrypoint.sh"]

Note that if you have an existing entrypoint, you will need to merge it with this one - you can only have one entrypoint.

Now you should find that the environment variable is respected i.e:

docker run -e CONFIG_VALUE=100 container_name cat CONFIG_FILE

Should work as expected.