Docker graph caching

DOCS.md

@@ -77,10 +77,36 @@ publish:
     build_args:
       - HTTP_PROXY=http://yourproxy.com
 ```
-
-## Layer Caching
 
-The Drone build environment is, by default, ephemeral meaning that you layers are not saved between builds. The below example combines Drone's caching feature and Docker's `save` and `load` capabilities to cache and restore image layers between builds:
+## Caching
+
+The Drone build environment is, by default, ephemeral meaning that you layers are not saved between builds. There are two methods for caching your layers.
+
+### Graph directory caching
+
+This is the preferred method when using the `overlay` or `aufs` storage drivers. Just use Drone's caching feature to backup and restore the directory `/drone/docker`, as shown in the following example:
+
+```yaml
+publish:
+  docker:
+    username: kevinbacon
+    password: pa55word
+    email: [email protected]
+    repo: foo/bar
+    tag:
+      - latest
+      - "1.0.1"
+
+cache:
+  mount:
+    - /drone/docker
+```
+
+NOTE: This probably won't work correctly with the `btrfs` driver, and it will be very inefficient with the `devicemapper` driver. Please make sure to use the `overlay` or `aufs` storage driver with this method.
+
+### Layer Caching
+
+The below example combines Drone's caching feature and Docker's `save` and `load` capabilities to cache and restore image layers between builds:
 
 ```yaml
 publish:

main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"bytes"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -93,7 +94,7 @@ func main() {
 	}
 
 	go func() {
-		args := []string{"daemon"}
+		args := []string{"daemon", "-g", "/drone/docker"}
 
 		if len(vargs.Storage) != 0 {
 			args = append(args, "-s", vargs.Storage)
@@ -228,6 +229,29 @@ func main() {
 		}
 	}
 
+	// Remove untagged images, if any
+	var outbuf bytes.Buffer
+	cmd = exec.Command("docker", "images", "-q", "-f", "dangling=true")
+	cmd.Stdout = &outbuf
+	cmd.Stderr = os.Stderr
+	trace(cmd)
+	err = cmd.Run()
+	if err != nil {
+		os.Exit(1)
+	}
+
+	if outbuf.Len() > 0 {
+		images := strings.Split(strings.TrimSpace(outbuf.String()), "\n")
+		cmd = exec.Command("docker", append([]string{"rmi"}, images...)...)
+		cmd.Stdout = os.Stdout
+		cmd.Stderr = os.Stderr
+		trace(cmd)
+		err := cmd.Run()
+		if err != nil {
+			os.Exit(1)
+		}
+	}
+
 	// Save to tarred image repository
 	if len(vargs.Save.File) != 0 {
 		// if the destination directory does not exist, create it