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/entrypoint.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # This is the entrypoint script for the dockerfile. Executed in the
 # container at runtime.