Adding a CA certificate to GitLab which is running in a podman container (also works with docker containers, just replace podman with docker):
- Login to the podman container 
- Copy/install the CA certificates (in this case Digi-Issuing-CA01-G3.pem & Digi-Root-CA01-G3.pem)
- Restart the podman container
Example
euprdgitlab655:~ #
euprdgitlab655:~ # podman exec -it gitlab /bin/bash
root@ad24f5df0102:/#
root@ad24f5df0102:/#
root@ad24f5df0102:/# ls /etc/gitlab/
gitlab-secrets.json  gitlab.rb  ssh_host_ecdsa_key  ssh_host_ecdsa_key.pub  ssh_host_ed25519_key  ssh_host_ed25519_key.pub  ssh_host_rsa_key  ssh_host_rsa_key.pub  ssl  trusted-certs
root@ad24f5df0102:/#
root@ad24f5df0102:/# ls /etc/gitlab/trusted-certs/
07ac5923.0  Digi-Issuing-CA01-G2.pem  Digi-Root-CA-G2.pem  e0c0effb.0
root@ad24f5df0102:/#
root@ad24f5df0102:/# ls -lah /etc/gitlab/trusted-certs/
total 8.0K
drwxr-xr-x 2 root root  101 Mar 27 12:44 .
drwxrwxr-x 4 root root  250 Oct 15  2024 ..
lrwxrwxrwx 1 root root   19 Mar 27 12:44 07ac5923.0 -> Digi-Root-CA-G2.pem
-rw-r--r-- 1 root root 2.6K Sep 14  2021 Digi-Issuing-CA01-G2.pem
-rw-r--r-- 1 root root 2.3K Sep 14  2021 Digi-Root-CA-G2.pem
lrwxrwxrwx 1 root root   24 Mar 27 12:44 e0c0effb.0 -> Digi-Issuing-CA01-G2.pem
root@ad24f5df0102:/#
root@ad24f5df0102:/#
root@ad24f5df0102:/# vi /etc/gitlab/trusted-certs/Digi-Root-CA-G3.pem
root@ad24f5df0102:/# vi /etc/gitlab/trusted-certs/Digi-Issuing-CA-G3.pem
root@ad24f5df0102:/#
root@ad24f5df0102:/# ls -lah /etc/gitlab/trusted-certs/
total 16K
drwxr-xr-x 2 root root  158 Jul 14 10:16 .
drwxrwxr-x 4 root root  250 Oct 15  2024 ..
lrwxrwxrwx 1 root root   19 Mar 27 12:44 07ac5923.0 -> Digi-Root-CA-G2.pem
-rw-r--r-- 1 root root 2.3K Jul 14 10:16 Digi-Issuing-CA-G3.pem
-rw-r--r-- 1 root root 2.6K Sep 14  2021 Digi-Issuing-CA01-G2.pem
-rw-r--r-- 1 root root 2.3K Sep 14  2021 Digi-Root-CA-G2.pem
-rw-r--r-- 1 root root 1.9K Jul 14 10:16 Digi-Root-CA-G3.pem
lrwxrwxrwx 1 root root   24 Mar 27 12:44 e0c0effb.0 -> Digi-Issuing-CA01-G2.pem
root@ad24f5df0102:/#
root@ad24f5df0102:/# exit
exit
euprdgitlab655:~ #
euprdgitlab655:~ #
euprdgitlab655:~ #
euprdgitlab655:~ # podman ps
CONTAINER ID  IMAGE                                                   COMMAND               CREATED       STATUS                PORTS                                                           NAMES
ad24f5df0102  reg.subdomain.domain.tld/gitlab/gitlab-ee:18.0.3-ee.0   /assets/wrapper       3 months ago  Up 3 weeks (healthy)  0.0.0.0:80->80/tcp, 0.0.0.0:443->443/tcp, 0.0.0.0:2222->22/tcp  gitlab
aa22bdf8c33a  docker.io/library/nginx:1.27.5                          nginx -g daemon o...  3 months ago  Up 3 weeks            0.0.0.0:8443->8443/tcp                                          nginx
euprdgitlab655:~ #
euprdgitlab655:~ # podman stop gitlab
WARN[0010] StopSignal SIGTERM failed to stop container gitlab in 10 seconds, resorting to SIGKILL
gitlab
euprdgitlab655:~ # podman ps
CONTAINER ID  IMAGE                                                   COMMAND               CREATED       STATUS                   PORTS                                                           NAMES
ad24f5df0102  reg.subdomain.domain.tld/gitlab/gitlab-ee:18.0.3-ee.0   /assets/wrapper       3 months ago  Up 2 seconds (starting)  0.0.0.0:80->80/tcp, 0.0.0.0:443->443/tcp, 0.0.0.0:2222->22/tcp  gitlab
aa22bdf8c33a  docker.io/library/nginx:1.27.5                          nginx -g daemon o...  3 months ago  Up 3 weeks               0.0.0.0:8443->8443/tcp                                          nginx
euprdgitlab655:~ #
euprdgitlab655:~ #
[...] *wait* [...]
euprdgitlab655:~ #
euprdgitlab655:~ # podman ps
CONTAINER ID  IMAGE                                                   COMMAND               CREATED       STATUS                  PORTS                                                           NAMES
ad24f5df0102  reg.subdomain.domain.tld/gitlab/gitlab-ee:18.0.3-ee.0   /assets/wrapper       3 months ago  Up 7 minutes (healthy)  0.0.0.0:80->80/tcp, 0.0.0.0:443->443/tcp, 0.0.0.0:2222->22/tcp  gitlab
aa22bdf8c33a  docker.io/library/nginx:1.27.5                          nginx -g daemon o...  3 months ago  Up 3 weeks              0.0.0.0:8443->8443/tcp                                          nginx
euprdgitlab655:~ #