Add note about JDK9

Author: jdleesmiller

developer-tools/java/chapters/ch03-build-image.adoc

@@ -115,6 +115,27 @@ busybox             latest              54511612f1c4        9 days ago
 
 === Create a simple Java application
 
+[NOTE]
+====
+If you are running OpenJDK 9, `mvn package` may http://openjdk.java.net/jeps/182[fail] with
+[source, text]
+----
+[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.1:compile (default-compile) on project helloworld: Compilation failure: Compilation failure:
+[ERROR] Source option 1.5 is no longer supported. Use 1.6 or later.
+[ERROR] Target option 1.5 is no longer supported. Use 1.6 or later.
+----
+
+You can add
+[source, xml]
+----
+  <properties>
+    <maven.compiler.source>1.6</maven.compiler.source>
+    <maven.compiler.target>1.6</maven.compiler.target>
+  </properties>
+----
+to the generated `pom.xml` to target 1.6 instead. See also the link:chapters/ch03-build-image-java-9.adoc[Build a Docker Image for Java 9] chapter.
+====
+
 Create a new Java project:
 
 [source, text]