Implement custom setup commands (Dockerfile's RUN)

[denzp]

Dockerfile statements:

RUN ["sudo", "chown", "-R", "rust:rust", "/rust-src"]
RUN apt-get install ffmpeg

Should have equivalent cargo-wharf metadata:

[package.metadata.wharf.builder]
setup-commands = [
  { command = ["sudo", "chown", "-R", "rust:rust", "/rust-src"] },
  { shell = "apt-get install ffmpeg" }
]

[package.metadata.wharf.output]
pre-install-commands = [
  { command = ["sudo", "chown", "-R", "rust:rust", "/rust-src"] },
  { shell = "apt-get install ffmpeg" }
]

post-install-commands = [
  { shell = "lld /bin/exec1"}
]