Update shebangs to take env into consideration

Some operating systems don’t have `bash` in `/bin`, so we should take the
`$PATH` into consideration for searching it.

Signed-off-by: Sascha Grunert <sgrunert@suse.com>

Closes: #1804
Approved by: TomSweeneyRedHat

docs/tutorials/01-intro.md

@@ -110,7 +110,7 @@ Let's try it out (showing the prompt in this example to demonstrate the differen
 
 Notice we have a `/usr/bin` directory in the newcontainer's image layer. Let's first copy a simple file from our host into the container. Create a file called runecho.sh which contains the following:
 
-    #!/bin/bash
+    #!/usr/bin/env bash
     for i in `seq 0 9`;
     do
     	echo "This is a new container from ipbabble [" $i "]"

docs/tutorials/03-on-build.md

@@ -119,7 +119,7 @@ result-container
 For this example the ONBUILD instructions in the primary container image will be used to copy a shell script and then run it in the secondary container image.  For the script, we'll make use of the shell script from the [Introduction Tutorial][].  First create a file in the local directory called `runecho.sh` containing the following:
 
 ```
-#!/bin/bash
+#!/usr/bin/env bash
 
 for i in `seq 0 9`;
 do