The "...local.yaml" docker-compose files offer a way to build working Zabbix environments from scratch in a very easy manner - by firing a single command.

In many companies there is no direct internet access available. Instead one has to use a HTTP proxy.

Unfortunately it's not just about adding some "*_proxy" environment variables as the SVN protocol cannot be tunneled out-of-the-box through an HTTP proxy.
Obvious options to me are either switching from SVN to HTTP(S) or making use of an HTTP CONNECT tunnel application inside the containers.