Change image used for test to avoid image name cache preventing proper pull

[rnorth]

To fix build errors like this one, which I believe is happening when alpine:latest is recorded as present in Testcontainers' image name cache:

Gradle Test Executor 1 > org.testcontainers.dockerclient.AmbiguousImagePullTest > testNotUsingParse FAILED
    org.testcontainers.containers.ContainerLaunchException: Container startup failed
        at org.testcontainers.containers.GenericContainer.doStart(GenericContainer.java:331)
        at org.testcontainers.containers.GenericContainer.start(GenericContainer.java:312)
        at org.testcontainers.dockerclient.AmbiguousImagePullTest.testNotUsingParse(AmbiguousImagePullTest.java:30)

        Caused by:
        org.rnorth.ducttape.RetryCountExceededException: Retry limit hit with exception
            at org.rnorth.ducttape.unreliables.Unreliables.retryUntilSuccess(Unreliables.java:88)
            at org.testcontainers.containers.GenericContainer.doStart(GenericContainer.java:324)
            ... 2 more

            Caused by:
            org.testcontainers.containers.ContainerLaunchException: Could not create/start container
                at org.testcontainers.containers.GenericContainer.tryStart(GenericContainer.java:498)
                at org.testcontainers.containers.GenericContainer.lambda$doStart$0(GenericContainer.java:326)
                at org.rnorth.ducttape.unreliables.Unreliables.retryUntilSuccess(Unreliables.java:81)
                ... 3 more

                Caused by:
                com.github.dockerjava.api.exception.NotFoundException: Status 404: {"message":"No such image: alpine:latest"}
                    at com.github.dockerjava.core.DefaultInvocationBuilder.execute(DefaultInvocationBuilder.java:241)
                    at com.github.dockerjava.core.DefaultInvocationBuilder.post(DefaultInvocationBuilder.java:125)
                    at com.github.dockerjava.core.exec.CreateContainerCmdExec.execute(CreateContainerCmdExec.java:33)
                    at com.github.dockerjava.core.exec.CreateContainerCmdExec.execute(CreateContainerCmdExec.java:13)
                    at com.github.dockerjava.core.exec.AbstrSyncDockerCmdExec.exec(AbstrSyncDockerCmdExec.java:21)
                    at com.github.dockerjava.core.command.AbstrDockerCmd.exec(AbstrDockerCmd.java:35)
                    at com.github.dockerjava.core.command.CreateContainerCmdImpl.exec(CreateContainerCmdImpl.java:595)
                    at org.testcontainers.containers.GenericContainer.tryStart(GenericContainer.java:408)
                    ... 5 more

[rnorth]

We never use the latest tag of this image anywhere else, so we can safely use it here without worrying about it being recorded in the cache of available images.