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/docker/daemon.json

@@ -0,0 +1,11 @@
+{
+    "storage-driver": "overlay2",
+    "log-driver": "journald",
+    "experimental": true,
+    "ip6tables": true,
+    "log-opts": {
+        "tag": "{{.ImageName}}/{{.Name}}"
+    },
+    "data-root": "/mnt/data/docker",
+    "deprecated-key-path": "/mnt/overlay/etc/docker/key.json"
+}

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

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