How can I pass heroku config vars to docker build? - docker

Question

I'm trying to deploy my rails app to the Heroku using heroku.yml. This is my yml setup
setup:
addons:
- plan: cleardb-mysql
as: DATABASE
build:
docker:
web: Dockerfile
config:
RAILS_ENV: development
DATABASE_URL: mysql2://abcdef:1234567#somewhere-someplace-123.cleardb.net/heroku_abcdefg123456
run:
web: bin/rails server -p $PORT -b 0.0.0.0
and I'm using MySQL as a database. And here's my Dockerfile that the Heroku is using to build the image.
FROM ruby:2.6.5-alpine
ARG DATABASE_URL
ARG RAILS_ENV
# Adding the required dependencies
# Installing Required Gems
# Other Configs...
# Copying Gem Files
COPY Gemfile Gemfile.lock ./
# Installing Gems
RUN bundle install --jobs=4 --retry=9
# Copying package and package.lock
COPY package.json yarn.lock ./
# Installing node_modules
RUN yarn
# Copy everything to the from current dir to container_root
COPY . ./
#Compiling assets
RUN bundle exec rake assets:precompile # this precompilation step need DATABASE_URL
CMD ["rails", "server", "-b", "0.0.0.0"]
This is working as expected but the problem is I've to pass the database string in the heroku.yml file. Is there a way I can reference the config vars that are declared in the Heroku?
I tried this but it's not working as the docs also saying that the config-vars declared in Heroku are not available at the build time.
setup:
addons:
- plan: cleardb-mysql
as: DATABASE
build:
docker:
web: Dockerfile
config:
RAILS_ENV: $RAILS_ENV
DATABASE_URL: $DATABASE_URL
run:
web: bin/rails server -p $PORT -b 0.0.0.0
What could be the possible workaround for this issue?

Answer 1

Without using the heroku.yml a possible solution is to include the env variable in the Dockerfile.
Example with Java code:
CMD java -Dserver.port=$PORT -DmongoDbUrl=$MONGODB_SRV $JAVA_OPTS -jar /software/myjar.jar
You need to build the image locally, push and release it into the Heroku Registry: when it runs the Config Vars (ie MONGODB_SRV) are injected