How do I pass build-time config to heroku container build scripts? - docker

Question

I have a Dockerfile, thusly:
# Use a multi-stage build to handle the compiling, installing, etc.
# STAGE 1: Install node_modules on a stretch container
FROM node:10-stretch as base
ARG APP_DIR=/node
EXPOSE 3000
WORKDIR $APP_DIR
RUN chown node:node $APP_DIR
COPY --chown=node:node package*.json ./
USER node
RUN ["npm", "install", "--no-optional"]
COPY --chown=node:node . .
# STAGE 2: Extend the base image as a builder image
FROM base as builder
ARG APP_NAME
ARG BASE_URL
ARG NODE_ENV
RUN npm run build -- --mode=production && rm -rf node_modules && npm install --no-optional --production
# STAGE 3: Copy the 'build' directory from previous stage and run in alpine
# Since this does not extend the base image, we need to set workdir, user, etc. again.
FROM node:10-alpine
ARG APP_DIR=/node
EXPOSE 3000
WORKDIR ${APP_DIR}
COPY --from=builder --chown=node:node $APP_DIR/dist .
COPY --from=builder --chown=node:node $APP_DIR/node_modules ./node_modules
USER node
CMD ["node", "./server"]
and what I'd like to do is have thos environment variables in stage 2 set by Heroku on build. If I set those values in the Dockerfile, they do end up being available in the build - but otherwise they don't seem to get passed the values from heroku.yml.
According to this documentation I should be able to set these values in heroku.yml, but thus far - I've been unsuccessful. My heroku.yml file looks like this:
build:
docker:
web: Dockerfile
config:
APP_NAME: My App
BASE_URL: /
NODE_ENV: production
What am I doing wrong?

Answer 1

I think it's necessary to repeat the ARG statements like this in order to have them accessible to each build stage:
# All `ARG`s must be referenced appear *before* the first `FROM`
ARG APP_NAME
ARG BASE_URL
ARG NODE_ENV
# STAGE 1: Install node_modules on a stretch container
FROM node:10-stretch as base
# Any subset of the `ARG`s can then be imported into the build stage
ARG APP_DIR
# STAGE 2: Extend the base image as a builder image
FROM base as builder
# Any subset of the `ARG`s can then be imported into the build stage
ARG APP_NAME
ARG BASE_URL
ARG NODE_ENV
# STAGE 3: Copy the 'build' directory from previous stage and run in alpine
FROM node:10-alpine
# Any subset of the `ARG`s can then be imported into the build stage
ARG APP_DIR
NOTE If you overwrite the values in any state, e.g. ARG APP_NAME=/node, this value will supersede the values you're obtaining from the environment (from the heroku.yml)