Pihole

You first need to free port 53 on the Pi :

sudo nano /etc/systemd/resolved.conf

[Resolve]
DNS=1.1.1.1
FallbackDNS=1.0.0.1
#Domains=
#DNSSEC=no
#DNSOverTLS=no
#MulticastDNS=yes
#LLMNR=yes
#Cache=yes
#DNSStubListener=yes
#DNSStubListenerExtra=
#ReadEtcHosts=yes
#ResolveUnicastSingleLabel=no
DNSStubListener=no

sudo ln -sf /run/systemd/resolve/resolv.conf /etc/resolv.conf

sudo reboot now

Then create ./Pihole/etc-pihole/ and ./Pihole/etc-dnsmasq.d/

Now you can Install Pi-hole

---
version: '3.3'
services:
  pihole:
    container_name: pihole
    image: pihole/pihole:latest
    ports:
      - 53:53/tcp #DNS Port
      - 53:53/udp #DNS Port
      #- 67:67/udp #DHCP Port
      - 83:80/tcp #Dashboard Port
      #- 443:443/tcp #Port 443 is to provide a sinkhole for ads that use SSL.
    environment:
      TZ: Europe/Paris
      WEBPASSWORD: 01664892ganjah12xxxxxx
      DNS1: 1.1.1.1
      DNS2: 1.0.0.1
      DNSSEC: 'true'
    volumes:
      - /srv/dev-disk-by-uuid-aeb64686-5ba2-4319-a690-3215ec812b5d/path/Files/Pihole/etc-pihole/:/etc/pihole/
      - /srv/dev-disk-by-uuid-aeb64686-5ba2-4319-a690-3215ec812b5d/path/Files/Pihole/etc-dnsmasq.d/:/etc/dnsmasq.d/
    cap_add:
      - NET_ADMIN
    restart: unless-stopped

You might want to restart your modem / router.

To undo the changes :

Edit /etc/systemd/resolved.conf

sudo nano /etc/systemd/resolved.conf

Comment out DNS=, FallbackDNS= and DNSStubListener=no

Remove the /etc/resolv.conf symbolic link:

sudo rm /etc/resolv.conf

Reboot.