[test][test_docker] Reset container memory limit to a higher value

This commit resets the container memory limit on the memory-limit test
to a higher value.  It turns out that Docker-18.09 is complaining it
cannot set the memory of the container in question to 4 Mb, possibly
because it starts up with more than that memory already (see discussion
about this here:
https://unix.stackexchange.com/questions/412040/cgroups-memory-limit-write-error-device-or-resource-busy).  Increasing the container memory to 10 Mb
and to the allocated array to 20 Mb makes the test work again.