Ubuntu 18.04: Customized list of GNOME Shell favorite apps not applied to new users

I’m trying to customize the list of GNOME Shell favorite apps that all new users will get by default in their dash in Ubuntu 18.04 Bionic Beaver. Based on instructions from the GNOME System Administration Guide, I created a profile and a keyfile to customize the favorite-apps key of GNOME Shell in a global system database.

sudo su
# Create profile "user"
cat>/etc/dconf/profile/user<<"eof"
user-db:user
system-db:local
eof
# Create keyfile for system database "local"
mkdir /etc/dconf/db/local.d
cat >/etc/dconf/db/local.d/01-favorite-apps<<"eof"
[org/gnome/shell]
favorite-apps=['firefox.desktop', 'thunderbird.desktop', 'emacs25.desktop']
eof

dconf update
exit

The problem is that when I create a new user and this user then logs in for the first time, a new user database seems to be created automatically in $HOME/.config/dconf/user which has Ubuntu’s default settings for the favorite-apps key. My global defaults from the system database /etc/dconf/db/local are ignored.

When launching dconf-editor as the new user after initial login, the favorite-apps key shows up as modified, but with Ubuntu’s default list (Ubiquity,Nautilus etc.):

I should add that the unexpected content of $HOME/.config/dconf/user does not come from /etc/skel. That directory only has a .profile, .bashrc, .bash_logout and examples.desktop – nothing GNOME related in there.

Only when the user (or an admin) manually deletes the $HOME/.config/dconf/user file, the global defaults from /etc/dconf/db/local will be applied the next time the user logs in.

What can I do to prevent this manual step and get the desired favorite apps right from start?