--docker-build flag seems to have no effect with Gradle plugin
Since 0.15.0 version, when buildNative gradle task is invoked it asks for GRAALVM_HOME environment variable regardless of whether it is being run with --docker-build=true or --docker-build=false

**Expected behavior**
I would expect the build to not require GRAALVM_HOME set when using docker image as a build environment


**Environment (please complete the following information):**
 - Output of `uname -a` or `ver`: **Linux localhost.localdomain 5.1.7-300.fc30.x86_64 #1 SMP Wed Jun 5 12:32:28 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux**
 - Output of `java -version`: **openjdk version "1.8.0_212"**
 - GraalVM version (if different from Java): **graalvm-ce-1.0.0-rc16**
 - Quarkus version or git rev: **0.16.1**

diff --git a/devtools/gradle/src/main/java/io/quarkus/gradle/tasks/QuarkusNative.java b/devtools/gradle/src/main/java/io/quarkus/gradle/tasks/QuarkusNative.java
index 40fe7119d11..8ec05064312 100644
--- a/devtools/gradle/src/main/java/io/quarkus/gradle/tasks/QuarkusNative.java
+++ b/devtools/gradle/src/main/java/io/quarkus/gradle/tasks/QuarkusNative.java
@@ -217,9 +217,6 @@ public void setEnableCodeSizeReporting(boolean enableCodeSizeReporting) {
     @Optional
     @Input
     public String getGraalvmHome() {
-        if (graalvmHome == null || graalvmHome.length() < 1)
-            throw new GradleException(
-                    "The GRAALVM_HOME environment variable need to be set to your GraalVM root directory to use native mode");
         return graalvmHome;
     }
 
['devtools/gradle/src/main/java/io/quarkus/gradle/tasks/QuarkusNative.java']