Use the GNU Affero GPL. AGPLv3 is in the words of the Free Software Foundation “a modified version of the ordinary GNU GPL version 3”. It has one key added freedom for the user (GPL and free software in general is always about the user). This translates to a requirement in the AGPLv3:
if you run a modified program on a server and let other users communicate with it there, your server must also allow them to download the source code corresponding to the modified version running there.
Read more on why the GNU Affero GPL is right for you and your customers at https://www.gnu.org/licenses/why-affero-gpl.en.html