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
 #

docs/reference/builder.md

@@ -1029,7 +1029,7 @@ the final executable receives the Unix signals by using `exec` and `gosu`
 commands:
 
 ```bash
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 if [ "$1" = 'postgres' ]; then

man/generate.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Generate man pages for docker/docker
 #

man/md2man-all.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 # get into this script's directory