common: Fix bash shell script invocation

`#!/usr/bin/env bash` should be used to avoid hard paths.

To address:

  dockcross-manylinux-x64: bad interpreter: /bin/bash^M: no such file or directory

imagefiles/dockcross

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 DEFAULT_DOCKCROSS_IMAGE=dockcross/base  # DO NOT MOVE THIS LINE (see entrypoint.sh)