Here's what worked for me:
Run the bitwardenrs container without SSL:
sudo docker pull bitwardenrs/server:raspberry
sudo docker run -d --name bitwarden --restart always -e ADMIN_TOKEN=your_admin_token -v /bw-data/:/data/ -p 80:80 bitwardenrs/server:raspberry
Proxy through Caddy (note that I'm using DuckDNS for DNS):
curl https://getcaddy.com | bash -s personal tls.dns.duckdns
nohup caddy -http-port 999 -conf /etc/caddy/Caddyfile -envfile /etc/caddy/CaddyEnvFile -agree -email YOUR_EMAIL@gmail.com &
(the port 999 is because we've already got docker container running on port 80)
Follow the examples here for the configuration: https://github.com/dani-garcia/bitwarden_rs/wiki/Proxy-examp...