Holy cow!
First, thank you, as always. I spent hours and hours researching and found the same info about something else running on port 53; then I spent hours trying to figure out how to figure out what, if anything, was running on port 53.
I just exported the config and removed everything that didn't relate to the the container.
I was about to post it and noticed that the the container envs key DNSMASQ_USER was displayed in the export as "DNSMASQ_USER "
There was an extra space!
Removed it, started the container, and I think it's working.
Here's the broken config:
/interface bridge
add name=bridge1
add name=docker
/interface bridge port
add bridge=docker interface=veth1
/interface veth
add address=10.0.0.2/24 gateway=10.0.0.1 name=veth1
/ip address
add address=192.168.2.4/24 comment=defconf interface=bridge1 network=\
192.168.2.0
add address=10.0.0.1/24 interface=docker network=10.0.0.0
/container mounts
add dst=/etc/pihole name=etc_pihole src=/disk1/etc
add dst=/etc/dnsmasq.d name=dnsmasq_pihole src=/disk1/etc-dnsmasq.d
/container
add envlist=pihole_envs interface=veth1 logging=yes mounts=\
etc_pihole,dnsmasq_pihole root-dir=disk1/pihole
/container config
set registry-url=https://registry-1.docker.io tmpdir=disk1/pull
/container envs
add key=TZ name=pihole_envs value=America/New_York
add key=WEBPASSWORD name=pihole_envs value=mikrotik
add key="DNSMASQ_USER " name=pihole_envs value=root
/ip dhcp-server network
add address=192.168.2.0/24 comment=defconf gateway=192.168.2.2 netmask=24
/ip dns
set cache-max-ttl=3w cache-size=16384KiB servers=192.168.2.2