Add tests coverage with a custom context

[baptistegrimaud]

Similar to what has been done in jahia-ee (see https://github.com/Jahia/jahia-ee/blob/master/docker-tests/dc-mariadb-galera-10.11-custom-context.yml), we should run our Cypress tests when Jahia is started with a custom context.

Things to consider:

• if possible, we should also test our utility commands (yarn dev and yarn deploy), but those would very likely not be executed by Cypress
• some URLs may have to be adjusted to use the custom context if defined
• should we have something similar for Luxe?
• impacts on the CI cost with additional executions? should we run them on PR systematically? once a week?

[romain-pm]

@danielaebenberger FYI

[GauBen]

Doing this is the solution to ensure #549 is working (and closing https://github.com/Jahia/jahia-private/issues/3491), added this issue as a child of the spike

[Fgerthoffert]

Blocked by PR: #562

[baptistegrimaud]

#560 requires Jahia/jahia-modules-action#290 to be merged first (and released as v2).

[x0h01]

✅️ Custom context tests execution is added to nightly build.
✅️ Build passed for both - root and custom contexts.