enable configuring app using env-vars instead (or together) of config.json

[chiptus]

deploying openminter with custom config requires forking it and adding my own config.json.
this also requires merging from the original repo whenever there's an update.

it would be easier to set a few env vars and deploying from the original repo.

we can alsoo add default values to some fields (e.g. tzkt URL)