feat(config): add DOCKER_IMAGE_SELECTION_MODE=fixed for deterministic pentest image

[LuckVd]

Description

Fixes #392

DOCKER_DEFAULT_IMAGE_FOR_PENTEST (and the DefaultImageForPentest template variable) was documented and configured as if it constrains which Docker image a flow's terminal container uses. In practice it was only injected as text into the image_chooser prompt (backend/pkg/templates/prompts/image_chooser.tmpl), and the LLM's raw text response was used directly as the image name (backend/pkg/providers/providers.go, NewFlowProvider) with no validation and no deterministic fallback. The issue reporter demonstrated that weaker/local models (e.g. qwen2.5:7b-instruct) reliably ignore the pentest-image guidance and return e.g. node:latest, silently running pentest flows in the wrong environment.

This PR implements the issue's preferred option 1 (deterministic mode switch) as an opt-in setting, keeping the default behavior unchanged:

Change

• backend/pkg/config/config.go: new DockerImageSelectionMode field (env DOCKER_IMAGE_SELECTION_MODE, default "llm").
• backend/pkg/providers/providers.go:
  • NewFlowProvider now selects the image deterministically when DOCKER_IMAGE_SELECTION_MODE=fixed: it skips the LLM call entirely and uses the configured pentest image.
  • New small helper resolvePentestImage(pentestImage, defaultImage) — lowercases/trims the configured pentest image and falls back to the default docker image when it is unset (mirrors the empty-value handling in pkg/docker/client.go).
  • Any mode value other than "fixed" keeps the existing LLM-based selection, so existing deployments are unaffected.
• backend/pkg/providers/image_selection_test.go: new unit tests covering the helper (configured image, case/whitespace normalization, empty fallback).
• backend/pkg/config/config_test.go: DOCKER_IMAGE_SELECTION_MODE added to the hermetic env-clearing list.
• .env.example: new documented setting.

Scope note

The LLM path itself is intentionally unchanged (no allow-list validation, no behavior change for default deployments); operators who want a hard guarantee set DOCKER_IMAGE_SELECTION_MODE=fixed. The issue's suggested option 2 (response allow-list validation) would be a behavioral change for existing deployments and is deliberately not included.

Type of Change

• 🐛 Bug fix (opt-in deterministic enforcement)

Related Issue

• Fixes [Bug]: DOCKER_DEFAULT_IMAGE_FOR_PENTEST is not enforced — image selection relies entirely on an unverified LLM response, with no deterministic fallback #392

Testing

• go test ./pkg/providers/ ./pkg/config/ -count=1 — ok, both packages pass (includes the 4 new TestResolvePentestImage cases: configured image, case/whitespace normalization, empty-pentest fallback to default image, normalized fallback).
• go vet ./pkg/providers/ ./pkg/config/ — clean
• gofmt -l pkg/providers/ pkg/config/ — no output
• git diff --check — clean
• Resource notes: full-repository go test ./... was not run in this environment (resource-constrained sandbox, 4 cores / 3.6GB RAM; build limited via GOMAXPROCS=2 GOFLAGS=-p=1 GOMEMLIMIT=1000MiB). The change is confined to pkg/providers + pkg/config + .env.example; both affected packages and their tests are exercised.
• Behavior verification of fixed mode is by code review (mode branch skips the only LLM call between image selection and container creation; image is stored on the flow provider and used verbatim, matching the previous path).

Checklist

• git diff --check origin/main...HEAD passes
• Changed source files are formatted
• Targeted tests for the changed packages/components pass
• Diff-scoped lint passes where applicable (go vet)
• Full-repository checks were run, or any unrelated/environment-dependent failures are documented above (documented: only affected packages tested due to sandbox resource limits)
• Self-reviewed the code
• Added/updated tests covering the change
• Breaking changes are clearly called out in the description above (none — new setting defaults to the existing behavior)