lint Dockerfile

[sinkcup]

https://hadolint.github.io/hadolint/

$ docker run -i hadolint/hadolint < src/stubs/Dockerfile 
-:5 DL4006 warning: Set the SHELL option -o pipefail before RUN with a pipe in it. If you are using /bin/sh in an alpine image or if your shell is symlinked to busybox then consider explicitly setting your SHELL to /bin/ash, or disable this check
-:7 DL3008 warning: Pin versions in apt get install. Instead of `apt-get install <package>` use `apt-get install <package>=<version>`
-:7 DL3015 info: Avoid additional packages by specifying `--no-install-recommends`
-:20 DL4006 warning: Set the SHELL option -o pipefail before RUN with a pipe in it. If you are using /bin/sh in an alpine image or if your shell is symlinked to busybox then consider explicitly setting your SHELL to /bin/ash, or disable this check