docker-proxy: mark linux only

Unlike docker (cli only) this probably won't work on darwin.

    github.com/docker/libnetwork/networkdb
    can't load package: package github.com/docker/libnetwork/ns: build constraints exclude all Go files in /private/tmp/nix-build-docker-proxy-7b2b1feb1de4817d522cc372af149ff48d25028e.drv-0/go/src/github.com/docker/libnetwork/ns

/cc ZHF #45961