Explain that Docker Desktop is required to run S3ObjectFetchingTest

[rtyley]

The S3ObjectFetchingTest 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
• On Developer Laptops, Docker Engine will be provided by Docker Desktop, which must be started.
  • Guardian Devs: We have licenses for Docker Desktop

Without Docker Desktop, alternatives like Colima can be explored, but see:

• [Enhancement]: Support different docker context testcontainers/testcontainers-java#5837 - testcontainers-java currently doesn't support different Docker contexts
• https://www.atamanroman.dev/colima-restcontainers-intellij-setup-on-macos/ - describes a multi-step recipe for getting this working
• How to configure Testcontainers to work with Colima as container runtime? testcontainers/testcontainers-java#5034

This PR adds a message explaining the problem when Docker Engine isn't available - the default error in this situation wasn't fully explanatory, and didn't explain that Guardian devs will probably want to use & start Docker Desktop:

[info] S3ObjectFetchingTest:
[info] com.gu.etagcaching.aws.sdkv2.s3.S3ObjectFetchingTest *** ABORTED ***
[info]   java.lang.IllegalStateException: Previous attempts to find a Docker environment failed. Will not retry. Please see logs and check configuration
[info]   at org.testcontainers.dockerclient.DockerClientProviderStrategy.getFirstValidStrategy(DockerClientProviderStrategy.java:229)
[info]   at org.testcontainers.DockerClientFactory.getOrInitializeStrategy(DockerClientFactory.java:154)
[info]   at org.testcontainers.DockerClientFactory.client(DockerClientFactory.java:196)
[info]   at org.testcontainers.DockerClientFactory$1.getDockerClient(DockerClientFactory.java:108)
[info]   at com.github.dockerjava.api.DockerClientDelegate.authConfig(DockerClientDelegate.java:109)
[info]   at org.testcontainers.containers.GenericContainer.start(GenericContainer.java:321)
[info]   at com.gu.etagcaching.aws.sdkv2.s3.S3ObjectFetchingTest.beforeAll(S3ObjectFetchingTest.scala:52)
[info]   at org.scalatest.BeforeAndAfterAll.liftedTree1$1(BeforeAndAfterAll.scala:212)
[info]   at org.scalatest.BeforeAndAfterAll.run(BeforeAndAfterAll.scala:210)
[info]   at org.scalatest.BeforeAndAfterAll.run$(BeforeAndAfterAll.scala:208)
[info]   ...