Enable service account keys to be provided in base64 (#70)

DOCS.md

@@ -12,6 +12,8 @@ Refer to GAE [Access Control](https://cloud.google.com/appengine/docs/flexible/p
 Use least permissible role for the tasks required.
 Typically for `action: deploy` this is `App Engine Admin` and `Cloud Build Service Account`.
 
+If the service account JSON key is provided in base64 format, the plugin will decode it internally.
+
 For example:
 
 ```yml

main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bytes"
+	"encoding/base64"
 	"encoding/json"
 	"fmt"
 	"io"
@@ -244,6 +245,13 @@ func configFromEnv(vargs *GAE, workspace *string) error {
 		vargs.Token = os.Getenv("GAE_CREDENTIALS")
 	}
 
+	if decodedToken, err := base64.StdEncoding.DecodeString(vargs.Token); err == nil {
+		// if no error then the token is base64 encoded (or empty)
+		vargs.Token = string(decodedToken)
+	} else {
+		fmt.Println("gae_credentials is not base64 encoded: skipping decode")
+	}
+
 	// Maps
 	dummyVargs := dummyGAE{}
 	addlArgs := os.Getenv("PLUGIN_ADDL_ARGS")

main_test.go

@@ -229,3 +229,22 @@ func TestSetupFile(t *testing.T) {
 	}
 
 }
+
+func TestEncodedToken(t *testing.T) {
+
+	os.Setenv("PLUGIN_GAE_CREDENTIALS", "ZW5jb2RlZCBzZXJ2aWNlIGFjY291bnQga2V5")
+
+	vargs := GAE{}
+
+	workspace := ""
+	configFromEnv(&vargs, &workspace)
+
+	expectedDecodedToken := "encoded service account key"
+	assert.Equal(t, expectedDecodedToken, vargs.Token)
+
+	nonEncodedToken := "non-encoded service account key"
+	os.Setenv("PLUGIN_GAE_CREDENTIALS", nonEncodedToken)
+
+	configFromEnv(&vargs, &workspace)
+	assert.Equal(t, nonEncodedToken, vargs.Token)
+}