Runner Not finding header files

The runner I currently have on my server cannot find header files even though the libraries are installed. I can successfully compile when I’m logged in via ssh, yet the runner cannot compile it. All include directories are correct as I’ve said I’ve been able to compile it on the server. Is there some environment variable I’m supposed to be setting for the runner?

Here is the output from the build:

$ make -j2
g++  -c -std=c++11  -Isrc -I/usr/include/SDL2 -o main.o src/main.cpp
g++  -c -std=c++11  -Isrc -I/usr/include/SDL2 -o window.o src/window.cpp
In file included from src/texture.h:4:0,
                 from src/window.h:4,
                 from src/window.cpp:1:
src/sdl_smart_ptr.h:4:17: fatal error: SDL.h: No such file or directory
 #include <SDL.h>
                 ^
compilation terminated.
make: *** [window.o] Error 1
Makefile:56: recipe for target 'window.o' failed
make: *** Waiting for unfinished jobs....
In file included from /usr/include/sched.h:32:0,
                 from /usr/include/pthread.h:23,
                 from /usr/include/x86_64-linux-gnu/c++/4.9/bits/gthr-default.h:35,
                 from /usr/include/x86_64-linux-gnu/c++/4.9/bits/gthr.h:148,
                 from /usr/include/c++/4.9/ext/atomicity.h:35,
                 from /usr/include/c++/4.9/bits/ios_base.h:39,
                 from /usr/include/c++/4.9/ios:42,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iostream:39,
                 from src/init.h:4,
                 from src/main.cpp:1:
src/time.h:4:17: fatal error: SDL.h: No such file or directory
 #include <SDL.h>
                 ^
compilation terminated.
Makefile:53: recipe for target 'main.o' failed
make: *** [main.o] Error 1

Runner config.toml

concurrent = 1

[[runners]]
  url = "https://gitlab.com/ci"
  token = "REDACTED"
  tls-skip-verify = false
  tls-ca-file = ""
  name = "DigitalOcean"
  executor = "shell"
  [runners.ssh]
  [runners.docker]
    image = ""
    privileged = false
  [runners.parallels]
    base_name = ""