In Docker containers you cannot run `sysctl`. So, it is not possible to set
`sysctl -q net.ipv4.conf.all.src_valid_mark=1` from inside the container.
However you can set it when creating the container, with the Docker option:
`--sysctl net.ipv4.conf.all.src_valid_mark=1`

This patch checks first if `net.ipv4.conf.all.src_valid_mark` is already set,
before trying to set it. Otherwise `sysctl` will fail in a docker container,
and `wg-quick` will fail to start the interface.