Prepare usage of build secrets (#303)

* sort .gitignore and add apt_auth.conf to it
* add helper to setup.sh to create apt_auth.conf (user still needs to enter their own credentials)
   * only create apt_auth.conf in make if it doesn't already exist
* add tooling to core dockerfile and makefile to use build secrets
* add ignore for hadolint as mounting syntax is currently not supported so far
* update to latest docker on travis for buildkit support
* documentation
* sort gitignore
* add dockerfile syntax definition for build args

.gitignore

@@ -1,14 +1,14 @@
-.vscode/settings.json
-**/.vscode
 !.travis.yml
-.travis/config.yml
-data/*
-dive.log
 .env
+.travis/config.yml
+.vagrant
+.vscode/settings.json
 *.env
-docker-compose.override.yml
+*.pem
+**/.vscode
+apt_auth.conf
 build.tags
 calendar/kopano-calendar-*/
-*.pem
-apt_auth.conf
-.vagrant
+data/*
+dive.log
+docker-compose.override.yml