Then it should work with this: https://join-lemmy.org/docs/administration/install_docker.html

Here are the commands, basically:

Installing docker

From this doc: https://docs.docker.com/engine/install/ubuntu/

• for pkg in docker.io docker-doc docker-compose podman-docker containerd runc; do sudo apt-get remove $pkg; done
• apt-get update
• apt-get install ca-certificates curl gnupg
• install -m 0755 -d /etc/apt/keyrings
• curl -fsSL https://download.docker.com/linux/ubuntu/gpg | gpg --dearmor -o /etc/apt/keyrings/docker.gpg
• chmod a+r /etc/apt/keyrings/docker.gpg
• echo "deb [arch="$(dpkg --print-architecture)" signed-by=/etc/apt/keyrings/docker.gpg] https://download.docker.com/linux/ubuntu "$(. /etc/os-release && echo "$VERSION_CODENAME")" stable" | tee /etc/apt/sources.list.d/docker.list > /dev/null
• apt-get update
• apt-get install docker-ce docker-ce-cli containerd.io docker-buildx-plugin docker-compose-plugin docker-compose

This series of commands installed an official docker repository in your system so docker itself is always up-to-date, Ubuntu gets updates like this kinda slowly.

Installing Lemmy

From this doc: https://join-lemmy.org/docs/administration/install_docker.html

• mkdir /opt/lemmy
• cd /opt/lemmy
• wget https://raw.githubusercontent.com/LemmyNet/lemmy-ansible/main/templates/docker-compose.yml
• wget https://raw.githubusercontent.com/LemmyNet/lemmy-ansible/main/examples/config.hjson -O lemmy.hjson
• wget https://raw.githubusercontent.com/LemmyNet/lemmy-ansible/main/templates/nginx_internal.conf
• wget https://raw.githubusercontent.com/LemmyNet/lemmy-ansible/main/examples/customPostgresql.conf
• Edit all the downloaded files (for example using nano) and change everything that starts with {{ and ends with }}
• mkdir -p volumes/pictrs
• chown -R 991:991 volumes/pictrs
• docker-compose up -d

Installing webserver

I chose Caddy, you can choose a different one but then you'll have to check on your own.

From this guide: https://caddyserver.com/docs/install#debian-ubuntu-raspbian

• apt install -y debian-keyring debian-archive-keyring apt-transport-https
• curl -1sLf 'https://dl.cloudsmith.io/public/caddy/stable/gpg.key' | gpg --dearmor -o /usr/share/keyrings/caddy-stable-archive-keyring.gpg
• curl -1sLf 'https://dl.cloudsmith.io/public/caddy/stable/debian.deb.txt' | tee /etc/apt/sources.list.d/caddy-stable.list
• apt update
• apt install caddy
• Edit the file /etc/caddy/Caddyfile (for example using nano /etc/caddy/Caddyfile)
• Paste this content (replace domain.tld with your Lemmy instance domain and the 1236 with the port you have configured in the docker-compose.yml file for {{ lemmy_port }}):

domain.tld {
        @http {
                protocol http
        }
        redir @http https://{host}{uri}
        reverse_proxy localhost:1236
}

Hope I haven't forgotten anything, feel free to ask.

Edit: After all the commands, restart caddy with service caddy restart.

No, most browsers will do what they’re told. At least with nginx, you have to set up a server to listen on port 80 and send a 301 permanently redirected message with the https URL.

Ah, maybe I wasn't clear enough, caddy doesn't need a configuration to redirect traffic from port 80 to port 443, it does it automatically: https://caddyserver.com/docs/automatic-https

If you want to configure an http only site you need to specifically configure it in your Caddyfile.
Here's an example for a reverse proxy

# We don't specify the protocol so Caddy assumes HTTPS and redirects any HTTP to secure.  
secure.example.com { reverse_proxy :<port> }

# HTTP is specified so Caddy won't redirect to port 443 nor generate a certificate.  
http://insecure.example.com { reverse_proxy :<port> }