secret handling documentation for env missing attribute keys

[betelgeuse]

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Description

https://docs.docker.com/reference/cli/docker/buildx/build/#secret

Under env type secret there's a text: "env - Secret environment variable. id used if unset, otherwise will look for src, source if id unset."

This implies that there's undocumented src and source attributes.

Expected behaviour

Document the src and source attributes

Actual behaviour

They are not documented under env

Buildx version

4b3c3c8

Docker info

No response

Builders list

N/A

Configuration

N/A

Build logs

No response

Additional info

buildx/docs/reference/buildx_build.md

Line 647 in 4b3c3c8

- `env` - Secret environment variable. `id` used if unset, otherwise will look for `src`, `source` if `id` unset.