How to get an environment variable value into Dockerfile during "docker build"?

I'm building a container for a ruby app. My app's configuration is contained within environment variables (loaded inside the app with dotenv).

One of those configuration variables is the public ip of the app, which is used internally to make links. I need to add a dnsmasq entry pointing this ip to 127.0.0.1 inside the container, so it can fetch the app's links as if it were not containerized.

I'm therefore trying to set an ENV in my Dockerfile which would pass an environment variable to the container.

I tried a few things.

ENV REQUEST_DOMAIN $REQUEST_DOMAIN
ENV REQUEST_DOMAIN `REQUEST_DOMAIN`

Everything passes the "REQUEST_DOMAIN" string instead of the value of the environment variable though. Is there a way to pass environment variables values from the host machine to the container?

You should use the ARG directive in your Dockerfile which is meant for this purpose.

The ARG instruction defines a variable that users can pass at build-time to the builder with the docker build command using the --build-arg <varname>=<value> flag.

So your Dockerfile will have this line:

ARG request_domain

or if you'd prefer a default value:

ARG request_domain=127.0.0.1

Now you can reference this variable inside your Dockerfile:

ENV request_domain=$request_domain

then you will build your container like so:

$ docker build --build-arg request_domain=mydomain Dockerfile

Note 1: Your image will not build if you have referenced an ARG in your Dockerfile but excluded it in --build-arg.

Note 2: If a user specifies a build argument that was not defined in the Dockerfile, the build outputs a warning:

[Warning] One or more build-args [foo] were not consumed.