Use /usr/bin/env bash in shebangs to make them universal (#2735)

Some systems (like nixos) don't have bash living at `/bin/bash` so using
`/usr/bin/env bash` we can make these scripts universal.

### Pull Request Checklist

<!-- Please read docs/CONTRIBUTING.md before submitting your pull
request -->

* [X] I have added added tests for PR _or_ I have justified why this PR
doesn't need tests.
* [x] Pull request includes a [sign
off](https://github.com/matrix-org/dendrite/blob/main/docs/CONTRIBUTING.md#sign-off)

Signed-off-by: `Dov Alperin <git@dov.dev>`

Signed-off-by: `Dov Alperin <git@dov.dev>`

build/docker/images-build.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 cd $(git rev-parse --show-toplevel)
 

build/docker/images-pull.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 TAG=${1:-latest}
 

build/docker/images-push.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 TAG=${1:-latest}