Update Dockerfile to pull SPIRV-Headers before building.
Update Dockerfile to pull SPIRV-Headers before building.

fuchsia-mirror/third_party-shaderc,antiagainst/shaderc,antiagainst/shaderc,fuchsia-mirror/third_party-shaderc,fuchsia-mirror/third_party-shaderc,dneto0/shaderc,dneto0/shaderc,dneto0/shaderc,dneto0/shaderc,antiagainst/shaderc,antiagainst/shaderc