Allow configuring `SECRET_KEY_BASE` via values file

Currently, SECRET_KEY_BASE is populated as:

  SECRET_KEY_BASE: {{ randAlphaNum 90 | toString | b64enc }}

I'm using API keys generated by plausible and it looks like they're using SECRET_KEY_BASE for keys signature. So when I'm updating to a newer release keys is getting regenerated and all old API keys are not valid anymore. It would be great to have an option to specify this via values.