Add reuse-container mode

[rsennewald]

Add a persistent container feature. When reuse-container: true
is set, the plugin keeps a container alive across job steps
instead of creating and destroying one each time.
Key behaviors:

• Derives a stable container name from the image name and agent
  spawn index (or uses an explicit reuse-container-name override).
• On each job, checks for an existing running container: if the
  image digest matches, attaches via docker exec; if the digest
  differs, prints a warning banner, removes the stale container,
  and creates a fresh one.
• New containers are started detached (docker run -d ... sleep infinity) with --rm suppressed, then the job command runs via
  docker exec.
• Environment variables are stripped from the creation docker run
  and injected per-exec only, preventing secrets from leaking
  between jobs that share the container.
• The pre-exit hook skips container cleanup when reuse-container
  is enabled so the container persists for subsequent jobs.
  Files changed:
• plugin.yml: new reuse-container and reuse-container-name opts
• lib/shared.bash: get_reuse_container_name,
  get_container_image_id, get_image_id helpers
• commands/run.sh: reuse-container branch with exec-arg extraction,
  digest validation, and detached container creation
• hooks/pre-exit: skip cleanup when reuse-container is set
• tests/reuse-container.bats: 11 bats tests covering creation,
  reuse, digest mismatch, stopped container, custom name, spawn
  index, env passthrough, env stripping, pre-exit, --rm absence,
  and exit code propagation