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 $REQUEST_DOMAIN ENV REQUEST_DOMAIN `REQUEST_DOMAIN`
Everything passes the "REQUEST_DOMAIN" string instead of the value of the environment variable though. Is there a way to pass environment variables values from the host machine to the container?
Dockerfile provides a dedicated variable type ENV to create an environment variable. We can access ENV values during the build, as well as once the container runs.
If you want to pass variables through the docker-compose process into any of the Dockerfiles present within docker-compose. yml , use the --build-arg parameter for each argument to flow into all of the Dockerfiles.
Fetch Using docker exec Command Here, we are executing the /usr/bin/env utility inside the Docker container. Using this utility, you can view all the environment variables set inside Docker containers.
You can use docker run --env-file [path-toenv-file] to provide the environment variables to the container from a . env file.
You should use the ARG
directive in your Dockerfile which is meant for this purpose.
The
ARG
instruction defines a variable that users can pass at build-time to the builder with the docker build command using the--build-arg <varname>=<value>
flag.
So your Dockerfile will have this line:
ARG request_domain
or if you'd prefer a default value:
ARG request_domain=127.0.0.1
Now you can reference this variable inside your Dockerfile:
ENV request_domain=$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.
So you can do: cat Dockerfile | envsubst | docker build -t my-target -
Then have a Dockerfile with something like:
ENV MY_ENV_VAR $MY_ENV_VAR
I guess there might be a problem with some special characters, but this works for most cases at least.
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