Added getFirstMappedPort method

[bsideup]

This change simplifies the usage for 90% cases where users have something like:

@Rule
GenericContainer container = new GenericContainer().withExposedPorts(1234)

// ...

container.getMappedPort(1234)

by introducing new method:

container.getFirstMappedPort()

and it is just a safer version of:

container.getMappedPort(container.getExposedPorts().get(0))

It helps to remove some boilerplate, as well as "magic number" duplication.

[rnorth]

🤔 is use of streams absolutely essential here? 😉

[bsideup]

Well, at least I wouldn't say it's a misuse :)

Also, I prefer findFirst + map over !isEmpty() + .get(0), less error prone if you ask me

[rnorth]

Seems a very good idea- as you say for most usages this is going to simplify usage.