Environment variables are declared with the ENV statement and are notated in the Dockerfile either with $VARIABLE_NAME or ${VARIABLE_NAME}.
Passing variables at build-time
The ENV instruction sets the environment variable to the value. The environment variables set using ENV will persist when a container is run from the resulting image. For example:
FROM node:9 ENV PORT 3000 ENV NODE_ENV development
The Dockerfile allows you to specify arguments at build-time. The ARG instruction defines a variable that users can pass at to the builder:
FROM node:9 ARG PORT ARG NODE_ENV
When building a Docker image from the command line, you can set those values using –build-arg:
docker build --tag webapp --build-arg PORT=3000 --build-arg NODE_ENV=development .
Executing commands using the shell
And, here is the secret ingredient. If the $NODE_ENV variable is set, then you can use the shell to run an NPM script:
FROM node:9
ARG PORT
ARG NODE_ENV
ENV PORT $PORT
ENV NODE_ENV $NODE_ENV
RUN mkdir -p /usr/app
WORKDIR /usr/app
RUN cd /usr/app
ADD . .
RUN npm install
RUN /bin/bash -c '[[ "${NODE_ENV}" == "production" ]] && npm run build:prod || npm run build:dev'
EXPOSE $PORT
CMD ["npm", "run", "start"]
Finally, you expose the port number and start the HTTP server.
That’s it! Thanks for reading and happy Dockering :)
Federico Cargnelutti 