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/etc/templates/app.ini

@@ -0,0 +1,24 @@
+[repository]
+ROOT = /data/git/repositories
+
+[repository.upload]
+TEMP_PATH = /data/gitea/uploads
+
+[server]
+APP_DATA_PATH = /data/gitea
+
+[database]
+HOST = mysql:3306
+PATH = /data/gitea/gitea.db
+
+[session]
+PROVIDER_CONFIG = /data/gitea/sessions
+
+[picture]
+AVATAR_UPLOAD_PATH = /data/gitea/avatars
+
+[attachment]
+PATH = /data/gitea/attachments
+
+[log]
+ROOT_PATH = /data/gitea/log