Infer entrypoint/cmd of image or introduce option to set one

[nextrevision]

With busybox, /bin/bash may not be a viable shell, so we need to either infer the command prefix from the image's ENTRYPOINT/CMD, or add an argument to override the default /bin/bash.

[cllunsford]

I introduced a change for #41 to set the default command entrypoint to /bin/sh instead of /bin/bash. This should resolve the missing entrypoint problems for most images.