Add ETH_PRIVATE_KEY to Docker entrypoint

[veaceslavdoina]

This is a quick PR to add ETH_PRIVATE_KEY to the Docker entrypoint for variables names consistency.

During transition period we can use both, PRIV_KEY and ETH_PRIVATE_KEY, to the pass Ethereum private key.

After we will change all the code to use new variable, the old one will be removed.

[veaceslavdoina]

Added, thanks

PRIV_KEY variable is deprecated and will be removed in the next releases, please use ETH_PRIVATE_KEY instead!
Private key set