utils/docker-run: use portable bash shebang

/usr/bin/bash does not exist on Ubuntu 2018.04. Use the more portable
alternative "#!/usr/bin/env bash" which we are already using in quite
a few scripts.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Fixes: 242e9d72e7d2 ("utils/docker-run: new script")
[yann.morin.1998@free.fr: use "#!/usr/bin/env bash", not "@!/bin/bash"]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

utils/docker-run

@@ -1,4 +1,4 @@
-#!/usr/bin/bash
+#!/usr/bin/env bash
 set -o errexit -o pipefail
 DIR=$(dirname "${0}")
 MAIN_DIR=$(readlink -f "${DIR}/..")