See Runtime config variables, a feature created specifically for this buildpack that eliminates the re-build step by injecting variables into the Javascript bundle as each Heroku dyno starts-up.

Thank you for your answer! In fact, I'm already using runtime config variables in my app and it works like a charm, but PUBLIC_URL is used in webpack.config.prod.js file (I ejected), and I'm not sure I can use it inside this file (I tried but it didn't works...)

I see what you mean @marinav. Unfortunately I do not know much about how PUBLIC_URL is implemented. Maybe there's a clean way to solve this 🤷‍♂

I imagine working around this by writing a .profile script that search and replaces the URLs as each dyno start up, much like Runtime config vars does.

I had this same issue, trying to promote a cloudfront URL through heroku pipelines. I ended up doing something very similar to what @mars described.

Usage:

PUBLIC_URL={{DYNAMIC_PUBLIC_URL}}
DYANMIC_PUBLIC_URL=https://my-environment-specific-public-url.com

• PUBLIC_URL is used at build time by CRA. index.html will contain the literal string {{DYNAMIC_PUBLIC_URL}}
• DYNAMIC_PUBLIC_URL is used at startup time to perform a find and replace on the string {{DYNAMIC_PUBLIC_URL}}, wherever it is found in index.html

.profile.d/inject-dynamic-public-url.sh

if [ -f "build/index.html" ]; then
  sed -iE "s,{{DYNAMIC_PUBLIC_URL}},$DYNAMIC_PUBLIC_URL,g" build/index.html
fi

You might also like to add an echo, following a similar convention to the create react app build pack's run time env vars echo:

TEMPLATE_STR='{{DYNAMIC_PUBLIC_URL}}'
if [ -f "build/index.html" ]; then
  echo "Injecting 'DYNAMIC_PUBLIC_URL' value '$DYNAMIC_PUBLIC_URL' into build/index.html by replacing the text '$TEMPLATE_STR' (from .profile.d/inject_dynamic_public_url.sh)"
  sed -iE "s,$TEMPLATE_STR,$DYNAMIC_PUBLIC_URL,g" build/index.html
fi

Logs:

2019-09-26T04:33:52.586116+00:00 heroku[web.1]: Starting process with command `bin/boot`
2019-09-26T04:33:54.427983+00:00 app[web.1]: Injecting 'DYNAMIC_PUBLIC_URL' value 'https://your-dynamic-public-url' into build/index.html by replacing the text '{{DYNAMIC_PUBLIC_URL}}' (from .profile.d/inject_dynamic_public_url.sh)
2019-09-26T04:33:54.825351+00:00 app[web.1]: Injecting runtime env into /app/build/static/js/0.6a50fe1b.chunk.js (from .profile.d/inject_react_app_env.sh)
2019-09-26T04:33:55.661811+00:00 app[web.1]: Injecting runtime env into /app/build/static/js/1.20ee43f4.chunk.js (from .profile.d/inject_react_app_env.sh)