Fix spring-boot#481: add JRE_HOME

* JRE_HOME is often used as a convention to convey the path of a JRE; in this case it's convenient to distinguish the JRE from a JDK (when both are required), since the PATH can insert the JRE_HOME/bin or JDK_HOME/bin in a non predictable order.
* Thanks to JRE_HOME, subsequent build steps will be able to use JRE_HOME/bin/java to make sure they pick a JRE and not a JDK
* in case there's just a JDK desired by the user, JRE_HOME will also be set, as the JDK_HOME

jre.go

@@ -93,6 +93,7 @@ func (j JRE) Contribute(layer libcnb.Layer) (libcnb.Layer, error) {
 
 		if IsBuildContribution(j.Metadata) {
 			layer.BuildEnvironment.Default("JAVA_HOME", layer.Path)
+			layer.BuildEnvironment.Default("JRE_HOME", layer.Path)
 		}
 
 		if IsLaunchContribution(j.Metadata) {

jre_test.go

@@ -212,6 +212,7 @@ func testJRE(t *testing.T, context spec.G, it spec.S) {
 		Expect(layer.LayerTypes.Build).To(BeTrue())
 		Expect(layer.LayerTypes.Cache).To(BeTrue())
 		Expect(layer.BuildEnvironment["JAVA_HOME.default"]).To(Equal(layer.Path))
+		Expect(layer.BuildEnvironment["JRE_HOME.default"]).To(Equal(layer.Path))
 	})
 
 	it("marks before Java 9 JRE layer for launch", func() {