Separate build of front-end assets to the standalone image

[jelen07]

Why

We are currently building front-end assets within PHP images. It would be great to put this into a separate dedicated image to keep the production one as small as possible.

Specification

1. Current dependency https://github.com/68publishers/docker/blob/master/php/8.1-unit/Dockerfile#L114
2. Current build https://github.com/68publishers/consent-management-platform/blob/main/Makefile#L52