PX4 builds and testing are spread out over multiple continuous integration services.
Travis-ci is responsible for the official stable/beta/development binaries that are flashable through QGroundControl. It currently uses GCC 4.9.3 included in the docker image px4io/px4-dev-base and compiles px4fmu-{v1, v2, v4}, mindpx-v2, tap-v1 with makefile target qgc_firmware.
Travis-ci also has a MacOS posix sitl build which includes testing.
Semaphore is primarily used to compile changes for the Qualcomm Snapdragon platform, but also serves as a backup to Travis-ci using the the same px4io/px4-dev-base docker image. In addition to the set of firmware compiled by Travis-ci, Semaphore also builds for the stm32discovery, crazyflie, runs unit testing, and verifies code style.
CircleCI tests the proposed next version of GCC to be used for stable firmware releases using the docker image px4io/px4-dev-nuttx-gcc_next. It uses the makefile target quick_check which compiles px4fmu-v4_default, posix_sitl_default, runs testing, and verifies code style.