Configurable default image

Currently the default image for Dragonfly Instances is hardcoded (see [1]).

For air-gapped setups it would be nice to replace the default value. Maybe configurable via command-line or enviornment?

Thanks!

[1]: https://github.com/dragonflydb/dragonfly-operator/blob/b07f733376e75f4a600345e538c27ff258502e6f/internal/resources/const.go#L36

Start with internal/resources/const.go at the linked default-image constant and trace where that value is used when creating Dragonfly Instances. Determine the project's existing configuration or command-line/environment entry points, then verify that an air-gapped deployment can override the default image and that the existing default remains unchanged.

Written by the indexing model from the issue text.