Support setting the --config flag via the DEVBOX_CONFIG env var

[mikeland73]

Summary

Fixes #245.

The --config flag lets users point devbox at a devbox.json that lives in a non-default location. Until now there was no way to set it from the environment, which is awkward in contexts where passing the flag to every invocation is repetitive — the motivating example in the issue is a Dockerfile:

ENV DEVBOX_CONFIG=.devcontainer
RUN devbox install   # picks up DEVBOX_CONFIG automatically

This PR defaults the --config flag to the DEVBOX_CONFIG environment variable when the flag isn't passed explicitly. An explicitly passed --config/-c flag still takes precedence over the env var.

Changes

• Add a DevboxConfig (DEVBOX_CONFIG) constant in internal/envir.
• Use os.Getenv(DEVBOX_CONFIG) as the default value for the --config flag in both register and registerPersistent, and mention the env var in the flag usage text.
• Add TestConfigFlagDefaultsToEnv covering: unset env, env-as-default, and explicit-flag-overrides-env.

Notes / scope

Issue #245 also mentions a --cache flag. devbox no longer exposes a --cache flag for the .devbox directory location (DEVBOX_CACHE today is used for a different purpose — the nixpkgs cache base URL), so this PR scopes the change to the --config flag, which is the part that's actionable against the current CLI. Happy to follow up if a configurable .devbox path is still desired (tracked separately in #244).

Testing

go test ./internal/boxcli/ -run TestConfigFlagDefaultsToEnv
go vet ./internal/boxcli/ ./internal/envir/

https://claude.ai/code/session_01UQSFvFRfQ4Ku4F4LXbKjFn

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Generated by Claude Code

[Copilot]

Pull request overview

Adds support for setting the --config flag via a new DEVBOX_CONFIG environment variable, so users can avoid passing --config repeatedly (e.g., in Dockerfiles). Explicit --config/-c still takes precedence.

Changes:

• Introduces envir.DevboxConfig (DEVBOX_CONFIG) constant.
• Uses os.Getenv(envir.DevboxConfig) as the default for --config in both register and registerPersistent, with usage text mentioning the env var.
• Adds TestConfigFlagDefaultsToEnv covering unset env, env-as-default, and explicit-flag-override cases.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
internal/envir/env.go	Adds DevboxConfig constant with documentation.
internal/boxcli/config.go	Reads DEVBOX_CONFIG as default for --config; extracts shared usage string.
internal/boxcli/config_test.go	New unit tests verifying env-var default and flag override behavior.

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[mohsenari]

Idk why cli tests fail for this PR, worth a retry