Bug 26617: Update INSTALL file to include koha-testing-docker and Gitlab links

Test plan:
1. View the INSTALL file in a text editor
2. The INSTALL file should reference koha-testing-docker and KohaDevBox
using Gitlab links

Signed-off-by: David Nind <david@davidnind.com>

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>

INSTALL

@@ -4,5 +4,6 @@ consult http://wiki.koha-community.org/wiki/Koha_on_ubuntu_-_packages
 For instructions on how to install Koha on Debian from packages, please
 consult https://wiki.koha-community.org/wiki/Debian
 
-If you are a developer, you should take a look at KohaDevBox
-https://github.com/digibib/kohadevbox
+If you are a developer, you should take a look at koha-testing-docker or KohaDevBox:
+https://gitlab.com/koha-community/koha-testing-docker
+https://gitlab.com/koha-community/kohadevbox