Convert script shebangs from "#!/bin/bash" to "#!/usr/bin/env bash"

This is especially important for distributions like NixOS where `/bin/bash` doesn't exist, or for MacOS users who've installed a newer version of Bash than the one that comes with their OS.

Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>

contrib/completion/bash/docker

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # bash completion file for core docker commands
 #