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

core/Dockerfile

@@ -1,3 +1,4 @@
+# syntax = docker/dockerfile:1.0-experimental
 ARG docker_repo=zokradonh
 FROM ${docker_repo}/kopano_base
 
@@ -39,9 +40,10 @@ LABEL maintainer=az@zok.xyz \
 SHELL ["/bin/bash", "-o", "pipefail", "-c"]
 
 # install Kopano Core and refresh ca-certificates
-# hadolint ignore=DL3015
-RUN \
-    # apt key has already been installed in base
+# hadolint currently does not understand the extended buildkit syntax https://github.com/hadolint/hadolint/issues/347
+# hadolint ignore=SC2215,DL3015
+RUN --mount=type=secret,id=repocred,dst=/etc/apt/apt_auth.conf \
+    # apt key for this repo has already been installed in base
     echo "deb [${KOPANO_REPOSITORY_FLAGS}] ${KOPANO_CORE_REPOSITORY_URL} ./" > /etc/apt/sources.list.d/kopano.list; \
     # install
     apt-get update && \