Add devcontainer configuration

We already have everything needed to work inside a container, with this
configuration file developing in moby is even easier: the IDE will ask
you if you want to run everything inside a container and set it up for
you. No need to know that you have to run "BIN_DIR=. make shell" any
more.

Signed-off-by: Djordje Lukic <djordje.lukic@docker.com>

.devcontainer/devcontainer.json

@@ -0,0 +1,13 @@
+{
+  "name": "moby",
+  "build": {
+    "context": "..",
+    "dockerfile": "../Dockerfile",
+    "target": "dev"
+  },
+  "workspaceFolder": "/go/src/github.com/docker/docker",
+  "workspaceMount": "source=${localWorkspaceFolder},target=/go/src/github.com/docker/docker,type=bind,consistency=cached",
+
+  "remoteUser": "root",
+  "runArgs": ["--privileged"]
+}