Adds support for saving/restoring image layers from archive

DOCS.md

@@ -9,6 +9,9 @@ The following parameters are used to configure this plugin:
 * `tag` - repository tag for the image
 * `insecure` - enable insecure communication to this registry
 * `storage_driver` - use `aufs`, `devicemapper`, `btrfs` or `overlay` driver
+* `archive` - save and restore image layers to/from a tarred archive
+    * `file` - absolute or relative path to archive file
+    * `tag` - limit archiving to these tag(s) (optional)
 
 The following is a sample Docker configuration in your .drone.yml file:
 
@@ -53,6 +56,27 @@ publish:
 
 Note that in the above example we quote the version numbers. If the yaml parser interprets the value as a number it will cause a parsing error.
 
+You may want to cache Docker image layers between builds to speed up the build process:
+
+```
+publish:
+  docker:
+    username: kevinbacon
+    password: $$DOCKER_PASSWORD
+    email: kevin.bacon@mail.com
+    repo: foo/bar
+    tag:
+      - latest
+      - "1.0.1"
+    archive:
+      file: docker/image.tar
+      tag: latest
+
+cache:
+  mount:
+    - docker/image.tar
+```
+
 ## Troubleshooting
 
 For detailed output you can set the `DOCKER_LAUNCH_DEBUG` environment variable in your plugin configuration. This starts Docker with verbose logging enabled.

main.go

@@ -5,12 +5,18 @@ import (
 	"io/ioutil"
 	"os"
 	"os/exec"
+	"path/filepath"
 	"strings"
 	"time"
 
 	"github.com/drone/drone-plugin-go/plugin"
 )
 
+type Archive struct {
+	File     string   `json:"file"`
+	Tag      StrSlice `json:"tag"`
+}
+
 type Docker struct {
 	Storage  string   `json:"storage_driver"`
 	Registry string   `json:"registry"`
@@ -24,6 +30,7 @@ type Docker struct {
 	File     string   `json:"file"`
 	Context  string   `json:"context"`
 	Dns      []string `json:"dns"`
+	Archive  Archive  `json:"archive"`
 }
 
 func main() {
@@ -59,6 +66,12 @@ func main() {
 	if vargs.Tag.Len() == 0 {
 		vargs.Tag = StrSlice{[]string{"latest"}}
 	}
+	// Archive file can be both a relative or absolute path
+	if len(vargs.Archive.File) != 0 {
+		if ! filepath.IsAbs(vargs.Archive.File) {
+			vargs.Archive.File = filepath.Join(workspace.Path, vargs.Archive.File)
+		}
+	}
 
 	go func() {
 		args := []string{"-d"}
@@ -125,6 +138,23 @@ func main() {
 	trace(cmd)
 	cmd.Run()
 
+	// Load archived image if exists
+	if len(vargs.Archive.File) != 0 {
+		if _, err := os.Stat(vargs.Archive.File); err != nil {
+			fmt.Printf("Archive %s does not exist. Building from scratch.\n", vargs.Archive.File)
+		} else {
+			cmd := exec.Command("/usr/bin/docker", "load", "-i", vargs.Archive.File)
+			cmd.Dir = workspace.Path
+			cmd.Stdout = os.Stdout
+			cmd.Stderr = os.Stderr
+			trace(cmd)
+			err := cmd.Run()
+			if err != nil {
+				os.Exit(1)
+			}
+		}
+	}
+
 	// Build the container
 	name := fmt.Sprintf("%s:%s", vargs.Repo, vargs.Tag.Slice()[0])
 	cmd = exec.Command("/usr/bin/docker", "build", "--pull=true", "--rm=true", "-f", vargs.File, "-t", name, vargs.Context)
@@ -165,6 +195,33 @@ func main() {
 		}
 	}
 
+	// Save the image to the archive
+	if len(vargs.Archive.File) != 0 {
+		// if the path's directory does not exist, create it
+		dir := filepath.Dir(vargs.Archive.File)
+		os.MkdirAll(dir, 0755)
+
+		cmd = exec.Command("/usr/bin/docker", "save", "-o", vargs.Archive.File)
+
+		// Limit save command to the given tag(s)
+		if vargs.Archive.Tag.Len() != 0 {
+			for _, tag := range vargs.Archive.Tag.Slice() {
+				name_ := fmt.Sprintf("%s:%s", vargs.Repo, tag)
+				cmd.Args = append(cmd.Args, name_)
+			}
+		} else {
+			cmd.Args = append(cmd.Args, vargs.Repo)
+		}
+
+		cmd.Dir = workspace.Path
+		cmd.Stdout = os.Stdout
+		cmd.Stderr = os.Stderr
+		trace(cmd)
+		err := cmd.Run()
+		if err != nil {
+			os.Exit(1)
+		}
+	}
 }
 
 // Trace writes each command to standard error (preceded by a ‘$ ’) before it