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Running Docker in LXC (Proxmox)

Running Docker inside an LXC (Linux Container) on Proxmox is a popular way to save resources compared to running a full VM. However, it adds a layer of complexity: "Container Inception."

The Golden Rule: Enable Nesting​

For Docker to run inside an LXC container, you must enable the "Nesting" feature.

In Proxmox:

  1. Select your LXC container.
  2. Go to Options -> Features.
  3. Check Nesting.
  4. Restart the container.

Without this, Docker daemon will fail to start.

Privileged vs. Unprivileged​

By default, LXC containers are unprivileged (safer). Docker works fine in unprivileged containers if Nesting and keyctl features are enabled.

Privileged​

If you need to access host hardware (like passing through a GPU or a USB Zigbee stick) easily, you might need a privileged container.

  • Warning: This reduces security isolation. If root in the container breaks out, they are root on the Proxmox host.

Storage Drivers & ZFS​

If your Proxmox uses ZFS, you might hit issues with Docker's storage driver. Docker usually prefers overlay2.

If you see errors related to storage driver or "backing filesystem is unsupported":

  1. The Fix: Create a dedicated volume for Docker storage.
  2. The Workaround: Force Docker to use the fuse-overlayfs driver (slower but compatible).

Edit /etc/docker/daemon.json inside the LXC:

{
  "storage-driver": "fuse-overlayfs"
}

(You may need to install fuse-overlayfs package first).

Cgroups v2​

Modern Docker relies on Cgroups v2. Ensure your Proxmox host is running a modern kernel (Proxmox 7/8 usually does).

If you encounter cgroup errors, verify that /sys/fs/cgroup is mounted correctly inside the LXC.

Too Much Configuration?​

LXC is great for efficiency, but debugging storage drivers and cgroups can be a time sink.

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