Add support for adding and dropping linux capabilities to/from containers.

[albers]

Creating containers with adjusted capabilities is a more secure alternative to using privileged mode.
I have a use case that forces me to use privileged mode instead of just adding one missing capability.
I feel testcontainers should support capabilities.

I placed the new code below the privileged mode pieces because they are functionally related.
In implementig, I followed #725, which added support for container labels.

[bsideup]

Hi @albers,

This looks like a very niche functionality to me, and given that we provide withCreateContainerCmdModifier that allows applying customization to the CreateContainerCmd object, I am not sure we need to add it as a first class API.

WDYT?

[albers]

@bsideup I agree. I did not find withCreateContainerCmdModifier in the docs (because I did not look for a generic solution). thanks for pointing me to it.