Support build scan link for CircleCI

Detects running on CircleCI and configures the build scan with a link
to the CI build job URL.

See gh-31

src/main/java/io/spring/ge/conventions/gradle/BuildScanConventions.java

@@ -36,6 +36,8 @@ class BuildScanConventions implements Action<BuildScanExtension> {
 
 	private static final String BAMBOO_RESULTS_ENV_VAR = "bamboo_resultsUrl";
 
+	private static final String CIRCLECI_BUILD_URL_ENV_VAR = "CIRCLE_BUILD_URL";
+
 	private final Map<String, String> env;
 
 	private final ProcessRunner processRunner;
@@ -98,7 +100,7 @@ private void tagCiOrLocal(BuildScanExtension buildScan) {
 	}
 
 	private boolean isCi() {
-		if (isBamboo() || isConcourse() || isJenkins()) {
+		if (isBamboo() || isCircleCi() || isConcourse() || isJenkins()) {
 			return true;
 		}
 		return false;
@@ -108,6 +110,10 @@ private boolean isBamboo() {
 		return this.env.containsKey(BAMBOO_RESULTS_ENV_VAR);
 	}
 
+	private boolean isCircleCi() {
+		return this.env.containsKey(CIRCLECI_BUILD_URL_ENV_VAR);
+	}
+
 	private boolean isConcourse() {
 		return this.env.containsKey("CI");
 	}
@@ -157,6 +163,9 @@ else if (isJenkins()) {
 				buildScan.link("CI build", buildUrl);
 			}
 		}
+		else if (isCircleCi()) {
+			buildScan.link("CI build", this.env.get(CIRCLECI_BUILD_URL_ENV_VAR));
+		}
 	}
 
 	private RunResult getBranch() {

src/test/java/io/spring/ge/conventions/gradle/BuildScanConventionsTests.java

@@ -81,6 +81,14 @@ void whenBambooResultEnvVarIsPresentThenBuildScanHasACiBuildLinkToIt() {
 		assertThat(this.buildScan.links).containsEntry("CI build", "https://bamboo.example.com");
 	}
 
+	@Test
+	void whenCircleBuildUrlEnvVarIsPresentThenBuildScanHasACiBuildLinkToIt() {
+		new BuildScanConventions(this.processRunner,
+				Collections.singletonMap("CIRCLE_BUILD_URL", "https://circleci.example.com/gh/org/project/123"))
+						.execute(this.buildScan);
+		assertThat(this.buildScan.links).containsEntry("CI build", "https://circleci.example.com/gh/org/project/123");
+	}
+
 	@Test
 	void whenJenkinsUrlAndBuildUrlEnvVarsArePresentThenBuildScanHasACiBuildLinkToBuildUrl() {
 		Map<String, String> env = new HashMap<>();