Restructured docker building

I have restructured the docker build process entirely, the binary gets
built outside of the docker build command, now we are managing all
dependencies with real Alpine packages and I have dropped features like
socat or the cron daemon.

Signed-off-by: Thomas Boerger <tboerger@suse.de>

docker/usr/bin/entrypoint

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+for FOLDER in /data/gitea/conf /data/gitea/log /data/git /data/ssh; do
+    mkdir -p ${FOLDER}
+done
+
+if [ $# -gt 0 ]; then
+    exec "$@"
+else
+    exec /bin/s6-svscan /etc/s6
+fi