Add TF_DATA_DIR parameter

DOCS.md

@@ -222,5 +222,7 @@ role_arn_to_assume
 root_dir
 : The root directory where the terraform files live. When unset, the top level directory will be assumed.
 
+data_dir
+: The directory where terraform put's it's data files. When unset, .terraform assumed. (TF_DATA_DIR)
 parallelism
 : The number of concurrent operations as Terraform walks its graph.

main.go

@@ -73,6 +73,11 @@ func main() {
 			Usage:  "The root directory where the terraform files live. When unset, the top level directory will be assumed",
 			EnvVar: "PLUGIN_ROOT_DIR",
 		},
+		cli.StringFlag{
+			Name:   "data_dir",
+			Usage:  "The directory where terraform put's it's data files. When unset, .terraform assumed. (TF_DATA_DIR)",
+			EnvVar: "PLUGIN_DATA_DIR",
+		},
 		cli.StringFlag{
 			Name:   "secrets",
 			Usage:  "a map of secrets to pass to the Terraform `plan` and `apply` commands. Each value is passed as a `<key>=<ENV>` option",
@@ -145,6 +150,7 @@ func run(c *cli.Context) error {
 			Sensitive:   c.Bool("sensitive"),
 			RoleARN:     c.String("role_arn_to_assume"),
 			RootDir:     c.String("root_dir"),
+			DataDir:     c.String("data_dir"),
 			Parallelism: c.Int("parallelism"),
 			Targets:     c.StringSlice("targets"),
 			VarFiles:    c.StringSlice("var_files"),

plugin.go

@@ -29,6 +29,7 @@ type (
 		Sensitive   bool
 		RoleARN     string
 		RootDir     string
+		DataDir     string
 		Parallelism int
 		Targets     []string
 		VarFiles    []string
@@ -71,6 +72,10 @@ func (p Plugin) Exec() error {
 		assumeRole(p.Config.RoleARN)
 	}
 
+	if p.Config.DataDir != "" {
+		os.Setenv("TF_DATA_DIR", p.Config.DataDir)
+	}
+
 	// writing the .netrc file with Github credentials in it.
 	err := writeNetrc(p.Netrc.Machine, p.Netrc.Login, p.Netrc.Password)
 	if err != nil {