Add simple docker prune functionality

[veerendra2]

No description provided.

[Copilot]

Pull request overview

This PR adds a Docker “prune” step to ComposeFlux’s reconciliation loop, extending the Docker Compose client to call Docker Engine prune APIs after stack reconciliation.

Changes:

• Extend pkg/dockercompose.Client with a Prune method and implement container/image/volume/network/build-cache pruning via the Docker Engine API client.
• Invoke Docker pruning from Reconciler.Sync() after stack pruning.
• Promote github.com/docker/docker to a direct dependency and update .gitignore.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 3 comments.

File	Description
pkg/dockercompose/compose.go	Adds Docker Engine client usage and implements Client.Prune() (containers/images/volumes/networks/build cache).
internal/reconcile/sync.go	Calls Docker prune at the end of each sync cycle.
go.mod	Makes github.com/docker/docker a direct requirement (previously indirect).
.gitignore	Normalizes .direnv entry and adds *playground* ignore pattern.