Allow image application directory to be configurable

An `applicationDirectory` option on the Maven
`spring-boot:build-image` goal and the Gradle `bootBuildImage` task
can be configured to set the location that will be used to upload
application contents to the builder image, and will contain the
application contents in the generated image.

Closes gh-34786

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/test/java/org/springframework/boot/maven/ImageTests.java

@@ -193,6 +193,14 @@ class ImageTests {
 		assertThat(request.getCreatedDate()).isEqualTo("2020-07-01T12:34:56Z");
 	}
 
+	@Test
+	void getBuildRequestWhenHasApplicationDirectoryUsesApplicationDirectory() {
+		Image image = new Image();
+		image.applicationDirectory = "/application";
+		BuildRequest request = image.getBuildRequest(createArtifact(), mockApplicationContent());
+		assertThat(request.getApplicationDirectory()).isEqualTo("/application");
+	}
+
 	private Artifact createArtifact() {
 		return new DefaultArtifact("com.example", "my-app", VersionRange.createFromVersion("0.0.1-SNAPSHOT"), "compile",
 				"jar", null, new DefaultArtifactHandler());