Hostname support for backup container

Hi 👋 ,

currently, stack-back identifies its own container by checking if the container ID starts with the hostname:

if container_data.get("Id").startswith(socket.gethostname()):
    self.this_container = Container(container_data)

This works fine when using Docker's default behavior (hostname = container ID prefix), but fails when a custom hostname is set , for example in docker-compose.yml:

services:
  backup:
    hostname: "${COMPOSE_PROJECT_NAME}-backup"

This results in the error:

ValueError: Cannot find metadata for backup container

Use Case

Setting a custom hostname like ${COMPOSE_PROJECT_NAME}-backup is useful for:

• Consistent identification across container restarts
• Better monitoring and metrics (e.g., in Prometheus/Grafana)
• More readable hostnames in logs and dashboards

Proposed Solution

Extend the container detection logic to support both the original ID-based method and hostname-based detection:

for container_data in all_containers:
    container_id = container_data.get("Id", "")
    container_hostname = container_data.get("Config", {}).get("Hostname", "")
    
    if container_id.startswith(socket.gethostname()) or container_hostname == socket.gethostname():
        self.this_container = Container(container_data)
        break

Benefits

• Maintains backward compatibility with existing setups
• Allows users to set custom hostnames for better monitoring
• No breaking changes to current functionality

Start in src/restic_compose_backup/containers.py around line 410, where the backup container is identified from Docker metadata. Reproduce the failure with the custom hostname shown in the issue, then verify that both the default ID-based setup and the hostname-based setup locate the backup container without the metadata error.