"Can't infer shell" in debian:12.10 docker image

[sdavids]

$ mkdir -p /tmp/test/.devcontainer && cd "$_/.."
$ cat <<'EOF' >.devcontainer/devcontainer.json
{
  "name": "fnm-test",
  "image": "debian:12.10",
  "features": {
    "ghcr.io/devcontainers-extra/features/apt-get-packages:1.0.7": {
      "packages": "curl,unzip"
    }
  }
}
EOF

Open /tmp/test/ in Visual Studio Code with the Visual Studio Code Dev Containers extension enabled.

Wait for the "Folder contains a Dev Container configuration file. Reopen folder to develop in a container " notification and select "Reopen in Container".

Wait for the container to be started.

Open a new terminal.

root@27081550cb70:/workspaces/test# echo $SHELL
/bin/bash
root@27081550cb70:/workspaces/test# readlink /proc/$$/exe
/usr/bin/bash
root@27081550cb70:/workspaces/test# curl -fsSL https://fnm.vercel.app/install | bash 
Checking dependencies for the installation script...
Checking availability of curl... OK!
Checking availability of unzip... OK!
Downloading https://github.com/Schniz/fnm/releases/latest/download/fnm-linux.zip...
######################################################################## 100.0%
Installing for Bash. Appending the following to /root/.bashrc:

# fnm
FNM_PATH="/root/.local/share/fnm"
if [ -d "$FNM_PATH" ]; then
  export PATH="$FNM_PATH:$PATH"
  eval "`fnm env`"
fi

In order to apply the changes, open a new terminal or run the following command:

  source /root/.bashrc
root@27081550cb70:/workspaces/test# source /root/.bashrc
error: Can't infer shell!
fnm can't infer your shell based on the process tree.
Maybe it is unsupported? we support the following shells:
* bash
* zsh
* fish
* powershell

---

Can be reproduced without using Dev Containers as well:

$ docker run --rm -it debian:12.10 bash
root@3240e2cf1000:/# apt-get update && apt-get install -y curl unzip
root@3240e2cf1000:/# curl -fsSL https://fnm.vercel.app/install | bash
...
root@3240e2cf1000:/# source /root/.bashrc
error: Can't infer shell!
fnm can't infer your shell based on the process tree.
Maybe it is unsupported? we support the following shells:
* bash
* zsh
* fish
* powershell