Support poor network environments.

[lvelvee]

It will download Tini first. However, downloading is difficult in poor network environments.

• Can we add some arguments for Docker build, such as --build-arg, so we can set the https_proxy?
• Can we add some RUN code before any code in the Dockerfile except for FROM?

https://github.com/replicate/cog/blob/a81c7a4338be7596e64f3c3efe363852d6a4193d/pkg/dockerfile/generator.go#L269-L275 https://github.com/replicate/cog/blob/a81c7a4338be7596e64f3c3efe363852d6a4193d/pkg/docker/build.go#L34-L39

[ZhouhaoJiang]

I have also encountered many network issues, but they are not very friendly to Chinese developers