Add instructions for securely passing existing key(s) to Docker image(s)

[jbowen93]

Related to both #39 and #40

Inspired by this discussion: https://github.com/celestiaorg/celestia-node/pull/293#pullrequestreview-842793624

There should be instructions for how a Celestia app or node can be setup using existing private keys in a secure manner for both Docker and K8s. This should utilize Docker Swarm Secrets or K8s Secrets.