Allow Garden to read variables from files

[baspetersdotjpeg]

docker-compose lets you specify an env_file to decouple variables from the main configuration. Would be handy to let Garden do the same and make it easier for users migrating from docker-compose to adopt.

[edvald]

That's a good idea. One thing I'm wondering: Would we merge the variables from the file with the other variables under ${local.env.<key>} or have a separate key? I could lean either way tbh.

[baspetersdotjpeg]

Perhaps we could merge by default, and optionally allow users to specify a different key? When merging under local.env, in case of duplicate keys, I feel variables set in a service's config should override those set in the env file.

Perhaps @mickeeyo could share his thoughts here?

[edvald]

I'd like to avoid multiple layers of optionality if possible. Also, variables set in a service's config have nothing to do with the local.env key, doesn't flow in that direction, so there's nothing to consider there really.

So it's only a question of whether we merge env files into local.env or make e.g. a separate local.env-file (or even just env-file) template key. I'm sorta leaning towards the former but still not decided.