add PACKER_PLUGIN_PATH for plugin discovery

config.go

@@ -123,7 +123,23 @@ func (c *config) Discover() error {
 		return nil
 	}
 
-	// First, look in the same directory as the executable.
+	// First, check whether there is a custom Plugin directory defined. This gets
+	// absolute preference.
+	if packerPluginPath := os.Getenv("PACKER_PLUGIN_PATH"); packerPluginPath != "" {
+		sep := ":"
+		if runtime.GOOS == "windows" {
+			// on windows, PATH is semicolon-separated
+			sep = ";"
+		}
+		plugPaths := strings.Split(packerPluginPath, sep)
+		for _, plugPath := range plugPaths {
+			if err := c.discoverExternalComponents(plugPath); err != nil {
+				return err
+			}
+		}
+	}
+
+	// Next, look in the same directory as the executable.
 	exePath, err := osext.Executable()
 	if err != nil {
 		log.Printf("[ERR] Error loading exe directory: %s", err)
@@ -133,7 +149,7 @@ func (c *config) Discover() error {
 		}
 	}
 
-	// Next, look in the plugins directory.
+	// Next, look in the default plugins directory inside the configdir/.packer.d/plugins.
 	dir, err := packer.ConfigDir()
 	if err != nil {
 		log.Printf("[ERR] Error loading config directory: %s", err)

website/source/docs/extending/plugins.html.md

@@ -51,6 +51,7 @@ Once the plugin is named properly, Packer automatically discovers plugins in
 the following directories in the given order. If a conflicting plugin is found
 later, it will take precedence over one found earlier.
 
+
 1.  The directory where `packer` is, or the executable directory.
 
 2.  The `$HOME/.packer.d/plugins` directory, if `$HOME` is defined (unix)
@@ -62,6 +63,14 @@ later, it will take precedence over one found earlier.
 
 5.  The current working directory.
 
+6. The directory defined in the env var `PACKER_PLUGIN_PATH`. There can be more
+than one directory defined; for example, `~/custom-dir-1:~/custom-dir-2`.
+Separate directories in the PATH string using a colon (`:`) on posix systems and
+a semicolon (`;`) on windows systems. The above example path would be able to
+find a provisioner named `packer-provisioner-foo` in either
+`~/custom-dir-1/packer-provisioner-foo` or
+`~/custom-dir-2/packer-provisioner-foo`.
+
 The valid types for plugins are:
 
 -   `builder` - Plugins responsible for building images for a specific

website/source/docs/other/environment-variables.html.md

@@ -39,6 +39,14 @@ each can be found below:
     connections on your local host. The default is 10,000. See the [core
     configuration page](/docs/other/core-configuration.html).
 
+-   `PACKER_PLUGIN_PATH` - a PATH variable for finding third-party packer
+plugins. For example: `~/custom-dir-1:~/custom-dir-2`.
+Separate directories in the PATH string using a colon (`:`) on posix systems and
+a semicolon (`;`) on windows systems. The above example path would be able to
+find a provisioner named `packer-provisioner-foo` in either
+`~/custom-dir-1/packer-provisioner-foo` or
+`~/custom-dir-2/packer-provisioner-foo`.
+
 -   `CHECKPOINT_DISABLE` - When Packer is invoked it sometimes calls out to
     [checkpoint.hashicorp.com](https://checkpoint.hashicorp.com/) to look for
     new versions of Packer. If you want to disable this for security or privacy