add support for build-args

DOCS.md

@@ -16,6 +16,7 @@ The following parameters are used to configure this plugin:
     * `destination` - absolute / relative destination path
     * `tag` - cherry-pick tags to save (optional)
 * `load` - restore image layers from the specified tar file
+* `build_args` - [build arguments](https://docs.docker.com/engine/reference/commandline/build/#set-build-time-variables-build-arg) to pass to `docker build`
 
 The following is a sample Docker configuration in your .drone.yml file:
 
@@ -60,6 +61,19 @@ 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.
 
+It's also possible to pass build arguments to docker:
+
+```yaml
+publish:
+  docker:
+    username: kevinbacon
+    password: pa55word
+    email: kevin.bacon@mail.com
+    repo: foo/bar
+    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:

main.go

@@ -37,6 +37,7 @@ type Docker struct {
 	Dns       []string `json:"dns"`
 	Load      string   `json:"load"`
 	Save      Save     `json:"save"`
+	BuildArgs []string `json:"build_args"`
 }
 
 func main() {
@@ -175,7 +176,11 @@ func main() {
 
 	// 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)
+	cmd = exec.Command("/usr/bin/docker", "build", "--pull=true", "--rm=true", "-f", vargs.File, "-t", name)
+	for _, value := range vargs.BuildArgs {
+		cmd.Args = append(cmd.Args, "--build-arg", value)
+	}
+	cmd.Args = append(cmd.Args, vargs.Context)
 	cmd.Dir = workspace.Path
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr