Respect docker being an alias

[paracycle]

There are setups where the docker executable is an alias to another CLI which is a drop in replacement for docker, like podman.

Currently, this tool does not pick up those kinds of aliases, and fails with:

$ ./rails-new foo
thread 'main' panicked at src/main.rs:26:10:
Failed to execute process: Os { code: 2, kind: NotFound, message: "No such file or directory" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I think this might be because we are spawning a new process and not launching the new process in sub-shell.