Make containers work with READ_ONLY file system

[Timshel]

In both main and sso-support this is due to the change in start.sh to allow supporting different front-end.

For main

Fix should be trivial since we should be able to use WEB_VAULT_FOLDER to switch between the different front-ends.

For sso-support (cf PR)

We modify the /web-vault/app/main.*.css to hide/show the SSO button.
Basic solution could be to try to make the modification and continue if it fails but with current frontend builds it would mean no sso button :(.
Of course with a modified front end build the button could be shown by default.

[Timshel]

For main testing now include the modification in docker-hub and ghcr.io.