fix: detect userns-remap and set correct containerd namespace

[VedantMadane]

Fixes #50

When Docker is configured with userns-remap, it uses a containerd namespace like moby-100000.100000 instead of the default moby. Previously, docker-pussh always started unregistry with the default moby namespace, so pushed images were invisible to docker images on the remote host.

What this does

1. Adds a detect_containerd_namespace function that runs docker info --format '{{.DockerRootDir}}' on the remote host
2. If the Docker Root Dir ends with a UID.GID component (e.g. /var/lib/docker/100000.100000), derives the containerd namespace as moby-100000.100000
3. Passes -e UNREGISTRY_CONTAINERD_NAMESPACE to the unregistry container
4. Adds UNREGISTRY_CONTAINERD_NAMESPACE to the help/usage text

The detection uses a ^[0-9]+\.[0-9]+$ regex on the basename rather than just checking != docker, to avoid false positives on non-standard Docker Root Dirs (e.g. Snap installations at /var/snap/docker/common/var-lib-docker).

Users can still override the namespace explicitly via the UNREGISTRY_CONTAINERD_NAMESPACE environment variable.

How I tested

Verified the logic against the scenarios from your root cause analysis:

• /var/lib/docker -> basename docker -> no match -> namespace stays moby
• /var/lib/docker/100000.100000 -> basename 100000.100000 -> matches ^[0-9]+\.[0-9]+$ -> namespace becomes moby-100000.100000