Type-casting issues with fluent interface

[haines]

The Container#self() method doesn't return the correct type for subclasses of implementations. For example, the following will not compile because withEnv returns a GenericContainer not a DockerComposeContainer:

@ClassRule
public static DockerComposeContainer environment =
    new DockerComposeContainer(new File("src/test/resources/compose-test.yml"))
            .withEnv("SOMETHING", "hello world");

[rnorth]

@haines I think that #238 covers this issue and also includes a workaround, so I'll close this if that's OK with you?

[haines]

Yep, that's fine!