Speed up C++ builds via mounting ccache directory as a docker volume

[kszucs]

The plan here is to use mount a volume for each builder under ~/.ccache similarly like the JS builder does. This would spare ~3 minutes out of the current ~6 minutes builds.

class ArrowCppTest(DockerBuilder):
    volumes = [
        lambda builder: f'{slugify(builder.name)}:/root/.ccache:rw'
    ]

@pitrou is it safe enough to use the same ccache directory between revisions, including that the dependencies are also evolving over time.

[pitrou]

@pitrou is it safe enough to use the same ccache directory between revisions

Probably? However, you probably need a different volume for each builder (I'm not sure concurrent access would work).

[kszucs]

One volume per builder is the original plan, just not sure about ccache "consistency over speed" capabilities.

[kszucs]

resolved by #135