v0.13.139

Changes

Adds Shutdown predicate (#1900)
Shutdown predicate evaluates to true if Skipper is shutting down after receiving
SIGTERM signal and can be used to implement customized healthcheck.
Updates #1899

Docker image

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

docker run -it registry.opensource.zalan.do/teapot/skipper:v0.13.139 skipper --help
# arm64
docker run -it registry.opensource.zalan.do/teapot/skipper-arm64:v0.13.139 skipper --help
# arm v7 32bit
docker run -it registry.opensource.zalan.do/teapot/skipper-armv7:v0.13.139 skipper --help