Changes in Traefik Docker Provider

[wollomatic]

Due to a change in how Traefik retrieves the Docker API version (traefik/traefik#12256), the Socket-Proxy configuration for Traefik must be updated to allow HEAD requests to /_ping:

'-allowHEAD=/_ping'

Otherwise, Traefik would fall back to API version 1.51, which would break the Docker provider on older Docker versions.