honor containers.conf http_proxy and env settings during build

[unlimitedsola]

What type of PR is this?

/kind bug

What this PR does / why we need it

This PR resolves an issue where podman build ignored containers.conf settings (http_proxy = false and [containers] env). Please also see the linked issue for more context.

1. Binds the --http-proxy CLI flag default in pkg/cli/common.go to Containers.HTTPProxy from containers.conf instead of hardcoding true. This makes build proxy flag defaults consistent with podman run, allowing http_proxy = false in containers.conf to disable host environment proxy passthrough by default.
2. Updates configureEnvironment() in run_common.go to inject environment variables defined under [containers] env in containers.conf into the transient container process spec (g.AddProcessEnv) during RUN steps. This fixes the regression from pre-6.0.x where containers.conf environment settings applied to build execution commands (e.g. package management and network requests).

How to verify it

1. Configure containers.conf:

   [containers]
   http_proxy = false
   env = [
     "http_proxy=http://host.containers.internal:1080",
   ]

2. Export host shell environment variables:

   export http_proxy=http://127.0.0.1:1080

3. Run build:

   podman build --no-cache -f - . << 'EOF'
   FROM alpine
   RUN env | grep -i proxy
   EOF

4. Verify output contains http_proxy=http://host.containers.internal:1080 instead of http://127.0.0.1:1080.

Which issue(s) this PR fixes

podman-container-tools/podman#29299

Special notes for your reviewer

g.AddProcessEnv in configureEnvironment() sets the process spec environment (g.Config.Process.Env) for transient container execution during RUN steps. It does not touch builder.OCIv1.Config.Env, ensuring that environment variables configured in containers.conf apply to build execution without polluting committed image layers.

Does this PR introduce a user-facing change?

Not sure if a release note is needed for regressions, but this does contain user-facing change:

Honor `containers.conf` `http_proxy` and `env` settings during image builds, aligning the behavior with `podman run` and fixed regression introduced in 6.0.x.

[simonbrauner]

I would suggest against silently ignoring the error, similarly to other calls of config.Default in the file.

if err != nil {
	return nil, fmt.Errorf("failed to get container config: %w", err)
}

[unlimitedsola]

Thanks for the feedback! I changed the function signature so that the error can now bubble up instead of silently ignored.

[nalind]

LGTM, but I expect you'll need to run make fmt to fix the linter formatter warning.

[TomSweeneyRedHat]

LGTM
once the gofmt issue on line 87 in the test is made happy

[nalind]

Please squash your commits before merging. Thanks!

[packit-as-a-service]

Ephemeral COPR build failed. @containers/packit-build please check.

[unlimitedsola]

Ephemeral COPR build failed. @containers/packit-build please check.

I've squashed and rebased my commits. I think this was caused by me force-pushed twice too quickly?