Improvements to the Dockerfile

[Guztaver]

I added the pkg-config that is missing in the Dockerfile, and make the git clone the repository direct from the image, so the user don't need to clone it manually and after that pulls inside the Dockerfile!

If the git change is too much, tell me that I remove it and keep only the dependecies update, because without it, it's not compiling at all.

[Guztaver]

I was thinking about adding something to the git clone section if this change goes forward. (I didn't add it because I don't know if it will be included)

The changes are basicly adds cache to the git, to make compile times less longers when used mutiple times in a row:

RUN --mount=type=cache,target=/opt/FEX \
  git clone --recurse-submodules https://github.com/FEX-Emu/FEX.git /opt/FEX; \
  cd /opt/FEX \ 
  git pull \

[Sonicadvance1]

Seems reasonable to me. I'm surprised this docker file works at all since it is random user's whims to implement it.