Add docker.df

src/docker.js

@@ -8,6 +8,7 @@ exports = module.exports = {
     ping,
 
     info,
+    df,
     downloadImage,
     createContainer,
     startContainer,
@@ -630,6 +631,12 @@ async function info() {
     return result;
 }
 
+async function df() {
+    const [error, result] = await safe(gConnection.df());
+    if (error) throw new BoxError(BoxError.DOCKER_ERROR, 'Error connecting to docker');
+    return result;
+}
+
 async function update(name, memory, memorySwap) {
     assert.strictEqual(typeof name, 'string');
     assert.strictEqual(typeof memory, 'number');