Update certificates to enable self-signed certs (#3708)

Why:

* We are using self-signed ssl certificates for internal services, which results in failures when gitea tries to communicate through webhooks with these. We would like to enable gitea to be able to use these certificates without having to build custom docker images.

How

* We add the internal certificates to /usr/local/share/ca-certificates on the host
* We read-only mount /usr/local/share/ca-certificates from the host to /usr/local/share/ca-certificates in the container 
* We do a update-ca-certificates in the alpine container before starting gitea

This should have no consequence for users that do not have the need to handle self-signed certificates, as update-ca-certificates should be idempotent.

docker/etc/s6/gitea/setup

@@ -1,5 +1,7 @@
 #!/bin/bash
 
+/usr/sbin/update-ca-certificates
+
 if [ ! -d /data/git/.ssh ]; then
     mkdir -p /data/git/.ssh
     chmod 700 /data/git/.ssh