[BUG] USER environment variable overridden by shell - rpc-username always set to "root"

[giovaboy]

Is there an existing issue for this?

• I have searched the existing issues

Current Behavior

The USER environment variable defined in the compose file is ignored. The settings.json is always written with rpc-username: "root" regardless of the value set in the compose environment, causing 401 Unauthorized errors when trying to authenticate with the custom username.

Expected Behavior

The USER environment variable should correctly set rpc-username in settings.json to the value defined in the compose file.

Steps To Reproduce

1. Set USER=myuser and PASS=mypassword in compose environment
2. Start the container
3. Check /config/settings.json → rpc-username is "root" instead of "myuser"
4. Try to authenticate with myuser/mypassword → 401 Unauthorized
5. Authenticating with root/mypassword works correctly

Environment

- OS: Debian 13 (Trixie)
- Docker install: official Docker repository
- Image version: 4.1.1-r1-ls338

CPU architecture

x86-64

Docker creation

services:
  transmission:
    image: lscr.io/linuxserver/transmission:latest
    container_name: transmission
    environment:
      - PUID=1000
      - PGID=1000
      - TZ=Europe/London
      - USER=myuser
      - PASS=mypassword
    volumes:
      - /path/to/config:/config
      - /path/to/downloads:/downloads
    ports:
      - 9091:9091
    restart: unless-stopped

Container logs

issue is reproducible by inspecting /config/settings.json after container start.

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[Roxedus]

It works in my test. Whats the output of

docker exec transmission /usr/bin/with-contenv echo $USER

Is the container started directly with compose?

[giovaboy]

Yes, the container is started via Docker Compose using the OpenMediaVault 8 Compose plugin (Debian 13 Trixie).
Output of the two commands:

Shell expansion on host side

docker exec transmission /usr/bin/with-contenv echo $USER
giovaboy

Expansion inside container

docker exec transmission /usr/bin/with-contenv sh -c 'echo $USER'
root

[Roxedus]

I forgot about the expansion 😅, good catch.
The fact that it says root makes me believe the user env is overridden somewhere.
Does a inspect reflect this?`

docker inspect transmission --format '{{ .Config.Env | json }}'

[giovaboy]

Here is the output:
["TZ=Europe/Rome","PASS=mypassword","USER=root","PUID=1001","PGID=100",...]
As you can see, USER=root is already present in the container environment at inspect time, even though the compose file defines USER=myuser.

[Roxedus]

So the problem starts at container creation. I believe the inspect also holds some metadata about where the actual compose file used is stored, this might be a avenue for you to explore. It would not surprise me if this the OMV's plugin doing (check other containers spun up by it, to se if USER is set there as well).

[giovaboy]

USER=root appears only in the transmission container, not in others. So it's not the OMV plugin.
The transmission.env file correctly contains USER=myuser:
USER=myuser
PASS=mypass
But docker inspect shows USER=root, meaning Docker itself is overriding the USER env variable during container creation — before the init script even runs.
The compose override file is empty/auto-generated by OMV.
This seems to be Docker overriding the USER environment variable with the actual running user of the container process, which takes precedence over the compose-defined value.

[Roxedus]

Could you add USER to a random container managed by OMV, and see if it also shows up as root (based on the theory that the reading of the .env file gets expanded funny).

I may not be the only one that has forgotten about shell variable expansions 😄.

[giovaboy]

Tested with another container (beets). Added USER=giovaboy to its env file and the inspect correctly shows USER=giovaboy. So OMV is passing the variable correctly.
beets → USER=giovaboy
transmission → USER=root

[giovaboy]

Confirmed the root cause and found a working solution.
Root cause:
This was actually my mistake. In my compose file I had USER=${USER} instead of a literal value. Docker Compose expands ${USER} using the shell environment variable, which has the highest precedence in variable interpolation. Since the process running docker compose runs as root, the shell USER is root, overriding the .env file value.
Workaround:
Use an intermediate variable that doesn't conflict with the reserved shell variable USER:

compose file

environment:

• USER=${RPC_USER}

.env file

RPC_USER=myuser