ci: speed up Docker script to build images (#156)
ci: speed up Docker script to build images (#156)

This reduces the number of layers in the image, and as each layer is
"saved" and "committed", the fewer layers the less I/O, the faster the
build process (and our CIs).
GoogleCloudPlatform/functions-framework-cpp,GoogleCloudPlatform/functions-framework-cpp,GoogleCloudPlatform/functions-framework-cpp,GoogleCloudPlatform/functions-framework-cpp