Upgrade testcontainers to 1.15.1

According to [1], "focuses on the stability and compatibility". Includes
bug fixes and an update of docker-java to 3.2.7, which gets reflected in
this change just as well.

[1] https://github.com/testcontainers/testcontainers-java/releases/tag/1.15.1

Change-Id: Ib9118fab1c4ce17e74507b521467f21e4f837050