Explain that Docker Desktop is required

Author: rtyley

aws-s3/aws-sdk-v2/src/test/scala/com/gu/etagcaching/aws/sdkv2/s3/S3ObjectFetchingTest.scala

@@ -8,10 +8,11 @@ import com.gu.etagcaching.aws.sdkv2.s3.ExampleParser.parseFruit
 import com.gu.etagcaching.aws.sdkv2.s3.S3ClientForS3Mock.createS3clientFor
 import com.gu.etagcaching.aws.sdkv2.s3.TestS3Objects.bucket
 import com.gu.etagcaching.aws.sdkv2.s3.response.Transformer.Bytes
-import org.scalatest.{BeforeAndAfter, BeforeAndAfterAll, OptionValues}
 import org.scalatest.concurrent.{IntegrationPatience, ScalaFutures}
 import org.scalatest.flatspec.AnyFlatSpec
 import org.scalatest.matchers.should.Matchers
+import org.scalatest.{BeforeAndAfterAll, OptionValues}
+import org.testcontainers.DockerClientFactory
 import software.amazon.awssdk.services.s3.S3AsyncClient
 import software.amazon.awssdk.services.s3.model.PutObjectRequest
 
@@ -31,9 +32,26 @@ case class TestS3Objects(id: Int) {
   val example: ObjectId = ObjectId(bucket, s"$id/path")
   val nonExistent: ObjectId = ObjectId(bucket, s"$id/nothing-should-be-here")
 }
-
 class S3ObjectFetchingTest extends AnyFlatSpec with Matchers with ScalaFutures with OptionValues with IntegrationPatience with BeforeAndAfterAll {
 
+  require(DockerClientFactory.instance().isDockerAvailable,
+    """This test uses S3Mock/testcontainers/Docker, and requires Docker Engine to be running on the machine.
+      |
+      |In GitHub Actions, Docker Engine will be provided by Docker Server, already running:
+      |- https://github.com/actions/runner-images/blob/13c1652ebc00463d77a4a8acef1a508bc6a606a1/images/ubuntu/Ubuntu2404-Readme.md?plain=1#L75C3-L75C16
+      |
+      |On Developer Laptops, Docker Engine will be provided by Docker Desktop, which must be started.
+      |- Guardian Devs: We have licenses for Docker Desktop, see https://docs.google.com/document/d/1mYVMUM3XIZGQ8LzcwSeufu5k8hxnIzadipg6sOMXi3g/edit?usp=sharing
+      |
+      |Without Docker Desktop, alternatives like Colima can be explored, but note that unfortunately:
+      |- https://github.com/testcontainers/testcontainers-java/issues/5837 - Testcontainers does not currently support different Docker contexts
+      |- https://www.atamanroman.dev/colima-restcontainers-intellij-setup-on-macos/ - describes a substantial recipe for getting this working
+      |- https://github.com/testcontainers/testcontainers-java/issues/5034
+      |
+      |""".stripMargin
+  )
+
+
   val s3Mock: S3MockContainer = new S3MockContainer("latest").withInitialBuckets(TestS3Objects.bucket)
   override def beforeAll(): Unit = s3Mock.start()
   override def afterAll(): Unit = s3Mock.stop()