In this case, you would want the Affero General Public License (AGPL) which specifically has a carve-out (in?) for web servers. Section 13, "Remote Network Interaction; Use with the GNU General Public License." [0]
[0] "[...] your modified version must prominently offer all users interacting with it remotely through a computer network (...) an opportunity to receive the Corresponding Source of your version"
[0] "[...] your modified version must prominently offer all users interacting with it remotely through a computer network (...) an opportunity to receive the Corresponding Source of your version"