create target based Dockerfile

Use docker --target to build selfhost or selfhost-privacy image. By default it builds ecloud

1. selfhost:

No privacy specific patches

2. ecloud:

Apply privacy based patches (used in ecloud)

Registry

Root Image: registry.gitlab.e.foundation/e/infra/ecloud/nextcloud/ - Root image

selfhost: registry.gitlab.e.foundation/e/infra/ecloud/nextcloud/selfhost:$TAG

ecloud: registry.gitlab.e.foundation/e/infra/ecloud/nextcloud/ecloud:$TAG