v0.13.6

Changes

Fix slack invite link (#1686)
Fixes #1685.

Docker image

Docker image is available in Zalando's Open Source registry:

docker run -it registry.opensource.zalan.do/teapot/skipper:v0.13.6 skipper --help
# arm64
docker run -it registry.opensource.zalan.do/teapot/skipper-arm64:v0.13.6 skipper --help
# arm v7 32bit
docker run -it registry.opensource.zalan.do/teapot/skipper-armv7:v0.13.6 skipper --help