Update testcontainers to a version that works with Docker v29

[ioannakok]

The etag-caching project has one test (S3ObjectFetchingTest) that depends on S3Mock/testcontainers-java and thus on Docker Containers. A recent version of Docker (v29, in November 2025) updated their Docker API requirement, meaning that S3ObjectFetchingTest fails like this on dev laptops using recent versions of Docker Desktop:

[info] com.gu.etagcaching.aws.sdkv2.s3.S3ObjectFetchingTest *** ABORTED ***
[info]   java.lang.IllegalStateException: Could not find a valid Docker environment. Please see logs and check configuration
[info]   at org.testcontainers.dockerclient.DockerClientProviderStrategy.lambda$getFirstValidStrategy$7(DockerClientProviderStrategy.java:274)
[info]   at java.base/java.util.Optional.orElseThrow(Optional.java:403)
[info]   at org.testcontainers.dockerclient.DockerClientProviderStrategy.getFirstValidStrategy(DockerClientProviderStrategy.java:265)
[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:38)
[info]   at org.scalatest.BeforeAndAfterAll.liftedTree1$1(BeforeAndAfterAll.scala:212)

Note that this is also the same failure we see when Docker Desktop is not running on the machine.

GitHub Actions also recently adopted this version of Docker:

• [Windows/Ubuntu] Docker Server and Client will be updated to version 29.1.*, Docker Compose will be updated to version 2.40.3 on February 9th, 2026 actions/runner-images#13474

See:

• Issue: [Bug]: Docker 29.0.0 could not find a valid Docker environment testcontainers/testcontainers-java#11212
• Fixed by: Set default docker API version to 1.44 testcontainers/testcontainers-java#11216
• Released in: v1.21.4 & v2.0.2

We're using S3Mock v4.11.0, which is the latest version of S3Mock that's been released or tagged. However, it looks like there has been some (unreleased) work on v5.0.0 of S3Mock:

• S3Mock 5.0.0 adobe/S3Mock#2453 - includes an update to testcontainers-java v2.0.2
• 5.0.0 maintenance adobe/S3Mock#2820

...when that new version of S3Mock is released, we won't need to explicitly force the testcontainers-java update anymore.

See also

• Explain that Docker Desktop is required to run S3ObjectFetchingTest #137