Use box-drawing characters in `docker images -tree`

This makes the output of `docker images -tree` look a little prettier.
Previously it displayed a combination of box-drawing characters and pipe
characters, so the lines didn't quite connect...

Before:

    └─aceb1e132fe5 Size: 487 MB (virtual 1.728 GB)
      |─c5480c55e00a Size: 44.89 MB (virtual 1.773 GB)
      | └─96c21b5e3c80 Size: 17.25 kB (virtual 1.773 GB)
      |   └─58f3f2293512 Size: 8.191 MB (virtual 1.782 GB)

After:

    └─aceb1e132fe5 Size: 487 MB (virtual 1.728 GB)
      ├─c5480c55e00a Size: 44.89 MB (virtual 1.773 GB)
      │ └─96c21b5e3c80 Size: 17.25 kB (virtual 1.773 GB)
      │   └─58f3f2293512 Size: 8.191 MB (virtual 1.782 GB)

commands.go

@@ -1237,9 +1237,9 @@ func (cli *DockerCli) WalkTree(noTrunc bool, images *[]APIImages, byParent map[s
 					cli.WalkTree(noTrunc, &subimages, byParent, prefix+"  ", printNode)
 				}
 			} else {
-				printNode(cli, noTrunc, image, prefix+"|─")
+				printNode(cli, noTrunc, image, prefix+"├─")
 				if subimages, exists := byParent[image.ID]; exists {
-					cli.WalkTree(noTrunc, &subimages, byParent, prefix+"| ", printNode)
+					cli.WalkTree(noTrunc, &subimages, byParent, prefix+"│ ", printNode)
 				}
 			}
 		}

docs/sources/commandline/cli.rst

@@ -553,15 +553,15 @@ Displaying image hierarchy
 
     $ sudo docker images -tree
 
-    |─8dbd9e392a96 Size: 131.5 MB (virtual 131.5 MB) Tags: ubuntu:12.04,ubuntu:latest,ubuntu:precise
+    ├─8dbd9e392a96 Size: 131.5 MB (virtual 131.5 MB) Tags: ubuntu:12.04,ubuntu:latest,ubuntu:precise
     └─27cf78414709 Size: 180.1 MB (virtual 180.1 MB)
       └─b750fe79269d Size: 24.65 kB (virtual 180.1 MB) Tags: ubuntu:12.10,ubuntu:quantal
-        |─f98de3b610d5 Size: 12.29 kB (virtual 180.1 MB)
-        | └─7da80deb7dbf Size: 16.38 kB (virtual 180.1 MB)
-        |   └─65ed2fee0a34 Size: 20.66 kB (virtual 180.2 MB)
-        |     └─a2b9ea53dddc Size: 819.7 MB (virtual 999.8 MB)
-        |       └─a29b932eaba8 Size: 28.67 kB (virtual 999.9 MB)
-        |         └─e270a44f124d Size: 12.29 kB (virtual 999.9 MB) Tags: progrium/buildstep:latest
+        ├─f98de3b610d5 Size: 12.29 kB (virtual 180.1 MB)
+        │ └─7da80deb7dbf Size: 16.38 kB (virtual 180.1 MB)
+        │   └─65ed2fee0a34 Size: 20.66 kB (virtual 180.2 MB)
+        │     └─a2b9ea53dddc Size: 819.7 MB (virtual 999.8 MB)
+        │       └─a29b932eaba8 Size: 28.67 kB (virtual 999.9 MB)
+        │         └─e270a44f124d Size: 12.29 kB (virtual 999.9 MB) Tags: progrium/buildstep:latest
         └─17e74ac162d8 Size: 53.93 kB (virtual 180.2 MB)
           └─339a3f56b760 Size: 24.65 kB (virtual 180.2 MB)
             └─904fcc40e34d Size: 96.7 MB (virtual 276.9 MB)