Get Environment Variable Value in Dockerfile

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.

An alternative using envsubst without losing the ability to use commands like COPY or ADD, and without using intermediate files would be to use Bash's Process Substitution:

docker build -f <(envsubst < Dockerfile) -t my-target .

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

Load environment variables from a file you create at runtime.

export MYVAR="my_var_outside"
cat > build/env.sh <<EOF
MYVAR=${MYVAR}
EOF

... then in the Dockerfile

ADD build /build
RUN /build/test.sh

where test.sh loads MYVAR from env.sh

#!/bin/bash
. /build/env.sh
echo $MYVAR > /tmp/testfile

If you just want to find and replace all environment variables ($ExampleEnvVar) in a Dockerfile then build it this would work:

envsubst < /path/to/Dockerfile | docker build -t myDockerImage . -f -