docs/rofl/features: Document public variables

[martintomazic]

Closes #2486.

Followed the style of secrets 1:1 (instructed codex to generate text for me and reviewed myself).

E.g. see https://explorer.oasis.io/testnet/sapphire/tx/0c788cbf3be706f6641e6d475d5af126c482e05e6c7dcd5e05e0930364a03db7 for real deployment.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.23%. Comparing base (3ba354d) to head (57f31ba).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2487      +/-   ##
==========================================
- Coverage   51.24%   51.23%   -0.02%     
==========================================
  Files         237      237              
  Lines       17294    17294              
==========================================
- Hits         8862     8860       -2     
- Misses       8372     8374       +2     
  Partials       60       60              

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[matevz]

Maybe add some alternatives such as Compose file and .env, for example:

Suggested change

	## Public variables vs Compose file and `.env`
	Apart from public variables ROFL developers can expose environment variables directly in the Compose file as literals. For example:
	```yaml
	services:
	test:
	image: docker.io/library/alpine:3.21.2@sha256:f3240395711384fc3c07daa46cbc8d73aa5ba25ad1deb97424992760f8cb2b94
	command: echo "API URL is $API_URL"
	environment:
	- API_URL="https://api.example.com/v1"
	```
	The downside of this approach is that each time the literal is updated, the hash of the Compose file changes and thus the ROFL needs to be rebuilt and redeployed.
	Another approach is storing public environment variables inside the `.env` file. In this case, users can investigate the container image to examine environment variables. This approach comes handy during development, because you can feed the same `.env` file locally and then inside production ROFL. However, similarly to the approach above, the container hash will change each time you change the `.env` file and consequently the container hash changes in your Compose file, which requires you to rebuild and redeploy ROFL.

[martintomazic]

Does this add value?

1. Compose file literals probably one can infer from the existing compose syntax.
2. Alternative doesn't have much to do with public variables, as one can bake arbitrary stuff into docker image, doesn't have to be .env, can be config.toml also.

This approach comes handy during development, because you can feed the same .env file locally and then inside production ROFL.

you can do the same by adding explicit --env-file flag to compose command.

[matevz]

Maybe rename to public_variables.md for consistency? The chapters here are named more SEO-like and not what CLI names the commands.