Missing dependency for compositor-proxy in Dockerfile

[rosensvv]

The dockerfile for compositor-proxy refused to build until I added libgl1-mesa-dev to the dependencies. The error it gave was:

CMake Error at /usr/share/cmake-3.24/Modules/FindPkgConfig.cmake:607 (message):
  A required package was not found
Call Stack (most recent call first):
  /usr/share/cmake-3.24/Modules/FindPkgConfig.cmake:829 (_pkg_check_modules_internal)
  CMakeLists.txt:22 (pkg_check_modules)

-- Configuring incomplete, errors occurred!
See also "/app/build/CMakeFiles/CMakeOutput.log".
error Command failed with exit code 1.