make docker-compose easier to use (also by introducing variables and a setup script)

update Dockerfiles/Makefile for use with kopano supported build
introduce a local ldap image with some demo users
include caddy for reverse proxying
add proper documentation and remove obsolete build and push scripts

Signed-off-by: Felix Bartels <felix@host-consultants.de>

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "caddy"]
+	path = caddy
+	url = https://github.com/fbartels/caddy-gen.git