v0.13.133

Changes

Updates operation.md X-Forwarded-Port docs (#1890)
Followup on #1862

Docker image

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

docker run -it registry.opensource.zalan.do/teapot/skipper:v0.13.133 skipper --help
# arm64
docker run -it registry.opensource.zalan.do/teapot/skipper-arm64:v0.13.133 skipper --help
# arm v7 32bit
docker run -it registry.opensource.zalan.do/teapot/skipper-armv7:v0.13.133 skipper --help