[FEAT] Add custom CA certificates

[jeremydelattre59]

Is your feature request related to a problem? Please describe.
Hello, I have my personal Ntfy server in HTTPS protocol with personal CA certificate.
Instead of disabling certificate checking, it is possible to add it. Below is what you need to do to make it work.

Describe the solution you'd like

• bind host directory with /usr/local/share/ca-certificates in the container (already doable for example with volume : ${CONFIG_DIRECTORY}/${COMPOSE_PROJECT_NAME}/ca:/usr/local/share/ca-certificates)
• launch "/usr/sbin/update-ca-certificates --fresh --verbose" at every start of the container. "--fresh" is necessary when the certificate is removed or replaced

[AnalogJ]

If you're using the Omnibus image, this is super easy since we use s6-overlay (which supports init processes).
Just write a init script, and mount it into the /etc/cont-init.d/ folder inside the container.

See https://github.com/AnalogJ/scrutiny/tree/master/rootfs/etc/cont-init.d for some of the existing init scripts.