Add Easypanel one-click deploy option to Docker deployment guide

[Ahson-Shaikh]

Hi — this is Ahson Shaikh, DevOps Engineer at Easypanel. Easypanel is a self-hosted Docker deployment platform, and EmailEngine has a one-click deployment template there: https://easypanel.io/templates/emailengine

While testing, I noticed the template's Redis service wasn't explicitly setting maxmemory-policy noeviction as your docs require — fixed that separately in our own template repo (easypanel-io/templates#1588) and redeployed to confirm it boots correctly with the fix in place.

Added a short Easypanel option to DOCKER_DEPLOYMENT.md for anyone who'd rather not manage docker-compose and Redis tuning themselves.

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