Get environment variable value in Dockerfile

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.

This is for those looking to pass env variable from docker-compose using .env file to dockerfile during build and then pass those args as env variable to container. Typical docker-compose file

services:
  web:
    build:
      context: ./api
      dockerfile: Dockerfile
      args:
        - SECRET_KEY=$SECRET_KEY
        - DATABASE_URL=$DATABASE_URL
        - AWS_ACCESS_KEY_ID=$AWS_ACCESS_KEY_ID

Pass the env variable present in .env file to args in build command. Typical .env file

SECRET_KEY=blahblah
DATABASE_URL=dburl

Now when you run docker-compose up -d command, docker-compose file takes values from .env file then pass it to docker-compose file. Now Dockerfile of web containes all those varibales through args during build. Now typical dockerfile of web,

FROM python:3.6-alpine

ARG SECRET_KEY
ARG DATABASE_URL
ARG AWS_ACCESS_KEY_ID
ARG AWS_SECRET_ACCESS_KEY
ARG AWS_BUCKET
ARG AWS_REGION
ARG CLOUDFRONT_DOMAIN

ENV CELERY_BROKER_URL redis://redis:6379/0
ENV CELERY_RESULT_BACKEND redis://redis:6379/0
ENV C_FORCE_ROOT true
ENV SECRET_KEY  ${SECRET_KEY?secretkeynotset}
ENV DATABASE_URL ${DATABASE_URL?envdberror}

Now we recieved those secret_key and db url as arg in dokcerfile. Now let's use those in ENV as ENV SECRET_KEY ${SECRET_KEY?secretkeynotset}. Now even docker container has those variables in it's environment. Remember not to use ARG $SECRET_KEY(which I did). It should be ARG SECRET_KEY

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.