Allow additional routes in entrypoint

[sistason]

Currently, the proxy is only configurable for default openslides.

In my use case, I use SaToSa (https://github.com/IdentityPython/SATOSA) to proxy SAML to OIDC, for which I need a subdomain (satosa.openslides.example.com) route to get the traffic to a container inserted via an dockre-compose.override.yaml.

That functionality would be a nice addition to the openslides-proxy, as configuring additional tools into the landscape of microservices of OS does not sound very "dirty hack" to me.

As merging configs seems to be only possible with Caddyfile, not already computed json, configuring a fixed/per-variable merge/append directory for the entrypoint looks like the viable solution?
I'd think either:

• ADDITIONAL_PROXY_CONFIG with a path (mounted into the container) to just feed line-by-line to jq_write at the end of the entrypoint
• ADDITIONAL_PROXY_CONFIGS with an array doing the same as above (although arrays are icky in bash)
• ADDITIONAL_PROXY_ROUTES of just raw json to append to ".apps.http.servers.srv0.routes. Possibly appended early so the entrypoint can template the variables in there.

What do you think?