Fix docker build

[kcinay055679]

To test the build by using the updated dockerfile run the following command:

docker build \ 
--build-arg TZ=Europe/Zurich \
--build-arg RAILS_ENV=production \
--build-arg RAKE_ENV=production \
--build-arg BUNDLE_WITHOUT='development:metrics:test' \
--build-arg BUILD_SCRIPT='npm install -g yarn && yarn set version 1.22.10' \
--build-arg POST_BUILD_SCRIPT='cd frontend && yarn install --no-progress && yarn build-prod && mv -v dist/* ../public/' \
--build-arg RUN_PACKAGES='git imagemagick libpq5' \
--build-arg SECRET_KEY_BASE='cannot-be-blank-for-production-env-when-building' \
--no-cache \
--progress=plain \
-t skills \
.