Configuring the secret_key should be done on the app object (does not change the behavior)

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam.alba@gmail.com> (github: samalba)
Auto-generate secret_key only with dev setting flavor

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam.alba@gmail.com> (github: samalba)