Move Docker configuration to daemon.json (#2116)

This is more readable than passing arguments to the daemon directly. It
also shortens the ExecStart command significantly, which is stored in
every log entry in systemd-journald.

buildroot-external/rootfs-overlay/etc/systemd/system/docker.service.d/hassos.conf

@@ -2,6 +2,4 @@
 RequiresMountsFor=/etc/docker /mnt/data /var/lib/docker
 
 [Service]
-ExecStart=
-ExecStart=/usr/bin/dockerd -H fd:// --storage-driver=overlay2 --log-driver=journald --experimental --ip6tables --log-opt tag="{{.ImageName}}/{{.Name}}" --data-root /mnt/data/docker
 OOMScoreAdjust=-400