This setup also includes a SOCKS server that proxies your traffic over the same VPN. I use a separate browser (librewolf) and set the SOCKS proxy to :2020 including sending DNS over SOCKS. That's because my country blocks piracy-related sites at the DNS level. If you don't need this, you can delete the socks section of the docker-compose file.

On my ubuntu laptop, I install transmission-remote-gtk in order to click on a magnet link and have it added. Otherwise you have to browse to the container's web interface, which gets tiresome.

I have this installed as a systemd service so it runs on boot. I use the systemd state and credential features as a safeguard against my own mistakes with permissions, but my long-term goal is to encrypt these files on disk. Linux can be pwned - I have read that around 35% of botnet nodes are linux (although these are presumably mostly weak IoT devices). The secondary benefit of the LoadCredential/CREDENTIALS_DIRECTORY mechanism is that it doesn't expose secrets as environment variables.

The p2p.service file needs to be in that path, but you can put the other files wherever you want.

Known issues / todo list

The socks proxy sometimes falls over, I haven't looked into why
The downloaded files will be owned by root, since that's what the container runs as

File contents

/root/.secrets/mullvad:

123456789
""

For mullvad, there is no password, only an account number. I believe that the empty quotes are necessary. This file should be owned by root and chmod 600; containing dir should be 700. Replace the account number with your own account, obvs!

/etc/systemd/system/p2p.service:

[Unit]
Description=p2p
Requires=docker.service multi-user.target
After=docker.service network-online.target dhcpd.service

[Service]
Restart=always
RemainAfterExit=yes
WorkingDirectory=/usr/local/bin/p2p
ExecStart=docker compose up --remove-orphans
ExecStop=docker compose down
LoadCredential=mullvad:/root/.secrets/mullvad
DynamicUser=yes
SupplementaryGroups=docker
StateDirectory=p2p
StateDirectoryMode=700

[Install]
WantedBy=multi-user.target

/usr/local/bin/p2p/docker-compose.yml:

***
version: "3.7"

services:
  p2p:
    restart: always
    container_name: p2p
    image: haugene/transmission-openvpn   # see also: https://www.nickkjolsing.com/posts/dockermullvadvpn/
    cap_add:
      - NET_ADMIN
    sysctls:
      - "net.ipv6.conf.all.disable_ipv6=0"  # ipv6 must be enabled for Mullvad to work
    volumes:
      - ${STATE_DIRECTORY:-./config/}:/config   # dir managed by systemd - but defaults to ./config if running interactively
      - ${CREDENTIALS_DIRECTORY:-.}/mullvad:/config/openvpn-credentials.txt:ro  # var populated by LoadCredential - but defaults to ./mullvad if running interactively
      - transmission:/data
      - transmission_incomplete:/data/incomplete
      - /my/directory/Downloads:/data/completed
    environment:
      - OPENVPN_PROVIDER=MULLVAD
      - OPENVPN_CONFIG=se_all  # sweden
      - LOCAL_NETWORK=192.168.1.0/24    # put your own LAN network here - in most cases it should end in .0/24
      - TRANSMISSION_WEB_UI=flood-for-transmission  # optional
    ports:
      - 9091:9091
      - 80:9091
      - 2020:2020

  socks:
    restart: always
    container_name: socks
    image: lthn/dante
    network_mode: "service:p2p"
    volumes:
      - ./sockd.conf:/etc/sockd.conf
    depends_on:
      - p2p

volumes:
  transmission:
    external: false
  transmission_completed:
    external: false
  transmission_incomplete:
    external: false

/usr/local/bin/p2p/sockd.conf:

logoutput: stderr
# debug: 2
internal: 0.0.0.0 port = 2020
external: tun0
external.rotation: route

clientmethod: none
socksmethod: username none

user.privileged: root
user.notprivileged: nobody
user.unprivileged: sockd

# Allow everyone to connect to this server.
client pass {
    from: 0.0.0.0/0 to: 0.0.0.0/0
    log: connect error  # disconnect
}

# Allow all operations for connected clients on this server.
socks pass {
    from: 0.0.0.0/0 to: 0.0.0.0/0
    command: bind connect udpassociate
    log: error  # connect disconnect iooperation
    #socksmethod: username
}
# Allow all inbound packets.
socks pass {
    from: 0.0.0.0/0 to: 0.0.0.0/0
    command: bindreply udpreply
    log: error  # connect disconnect iooperation
}

Steps

Install docker and docker-compose, e.g. with sudo apt-get install -y docker-ce docker-ce-cli containerd.io docker-buildx-plugin docker-compose-plugin
Create the files with contents as above
sudo systemctl enable p2p
sudo systemctl start p2p
Check what it's doing: systemctl status p2p
On first start, it will take a few minutes to pull the images
To debug interactively while also passing the creds, use sudo systemd-run -P --wait -p LoadCredential=mullvad:/root/.secrets/mullvad docker compose up --remove-orphans
Every so often, cd into /usr/local/bin/p2p and run docker compose pull to update the images.