Add support for environmental secrets

[breca]

This merge adds support for type=env secret parameter, picking up the work of @71ms1 in #671.

It also adds some basic tests to the existing secret checks:

[test] podman start -a secrets_test_1
[test] | -rw-rw-r--    1 root     root            30 Feb 24 08:15 /run/secrets/custom_name
[test] | -rw-rw-r--    1 root     root            30 Feb 24 08:15 /run/secrets/file_secret
[test] | -r--r--r--    1 root     root             4 Feb 24 10:10 /run/secrets/my_secret
[test] | -r--r--r--    1 root     root             4 Feb 24 10:10 /run/secrets/my_secret_2
[test] | -r--------    1 103      103              4 Feb 24 10:10 /run/secrets/my_secret_3
[test] | -rw-rw-r--    1 root     root            30 Feb 24 08:15 /run/secrets/unused_params_warning
[test] | -rw-rw-r--    1 root     root            30 Feb 24 08:15 /etc/custom_location
[test] | important-secret-is-important
[test] | important-secret-is-important
[test] | sec
[test] | sec
[test] | sec
[test] | important-secret-is-important
[test] | important-secret-is-important
[test] | ENV_SECRET=sec
[test] exit code: 0

[p12tic]

The code looks good, thanks!

The PR needs unit tests, not just an integration test. In few weeks we will hopefully have an easy way to add unit tests to changes like this.

[breca]

Cool, happy to look at it when the bar is set.

[p12tic]

I looked into this again and think that we could simply have a unit test for get_secret_args function.

We already have tests for container_to_args() in pytests/test_container_to_args.py. At the moment splitting tests by function seems a bit non-optimal, but I expect we will have hundreds of lines of unit tests for each function. Splitting tests by function seems like an easy way to help navigating them.