Fixed issue where x265.pc file would be missing if git isn't installed

[throttlehead-dev]

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Took me awhile to track this one down but there's an issue where the x265.pc file isn't generated if git isn't installed.

• Added new --skip-ffmpeg-build option to skip ffmpeg build and just build the other libs. This made debugging the build happening inside a container little easier because I could split my command into two different statements in the Dockerfile.
• Added git install to this projects Dockerfile. It's not needed but it would be if this was compiling with --enable-gpl-and-non-free. If someone uses this Dockerfile as a starting point it's now obvious that git is needed.
• I also added a blurb about this to the Readme.