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/usr/lib/systemd/system/etc-docker.mount

@@ -1,14 +0,0 @@
-[Unit]
-Description=Docker persistent configuration
-Requires=mnt-overlay.mount
-After=mnt-overlay.mount
-Before=docker.service
-
-[Mount]
-What=/mnt/overlay/etc/docker
-Where=/etc/docker
-Type=none
-Options=bind
-
-[Install]
-WantedBy=hassos-bind.target