Add instruction to copy db.env.example to db.env in README

[guillermodlpa]

Looks like db.env is required for the fluxday-db image to build, as indicated on the docker-compose.yml file.

Without copying db.env.example to db.env, I would get this error:

This PR adds an instruction to the README to copy the file.

cc @ismudnx @stpnlr @halissonvit

Thanks!