Add format check to CI

[RobotSail]

We want to add yarn format:check as a step in the CI. We have added this command in PR #39, but ran into problems when enabling this check, where the local next.config.js passed the linter errors, but failed for some reason in CI.

[Gregory-Pereira]

/assign

[Gregory-Pereira]

With Tmate ssh I was able to determine that this comes from the Setup Pages step, where basePath is injected into the next.config.js which breaks the formatting. After discussing with @RobotSail, we decided to remove the base path injection and parameterize it or set it in ENV.