support/testing: build a glibc toolchain for docker / docker-compose tests

support/testing: build a glibc toolchain for docker / docker-compose tests

runc (which is a reverse dependency of docker-engine) is about to gain a
!uclibc dependency, so move to a glibc toolchain instead.

There are currently no prebuilt x86_64 / core2 / glibc toolchains available,
so instead use the internal toolchain backend to build one.

While we are at it, drop the infra.basetest.BASIC_TOOLCHAIN_CONFIG
reference, as that ARM toolchain configuration doesn't make any sense for
this x86-64 based test.

add docker / docker-compose tests

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>