Issue building in sim mode

Hi,

When I try to build my docker image with SGX_MODE=SIM, I get the following error:

```
/usr/bin/ld: warning: libsgx_uae_service_sim.so, needed by /opt/intel/sgxsdk/lib64/libsgx_urts_sim.so, not found (try using -rpath or -rpath-link)
/opt/intel/sgxsdk/lib64/libsgx_urts_sim.so: undefined reference to `get_launch_token'
collect2: error: ld returned 1 exit status
Makefile:235: recipe for target 'app' failed
make[1]: Leaving directory '/opt/intel/sgxsdk/SampleCode/SampleEnclave'
make[1]: *** [app] Error 1
Makefile:180: recipe for target 'all' failed
make: *** [all] Error 2
The command '/bin/sh -c make SGX_MODE=SIM' returned a non-zero code: 2
```

it seems that ubuntu can't find the sdk path. what should i add to my docker file to solve this issue?

thank you!