fix(backend/build): Update stale Poetry usage in Dockerfile (#12124)

[SECRT-2006: Dev deployment failing: poetry not found in container
PATH](https://linear.app/autogpt/issue/SECRT-2006)

- Follow-up to #12090

### Changes 🏗️

- Remove now-broken Poetry path config values
- Remove usage of now-broken `poetry run` in container run command
- Add trigger on `backend/Dockerfile` changes to Frontend CI workflow

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - If it works, CI will pass

.github/workflows/platform-frontend-ci.yml

@@ -6,10 +6,12 @@ on:
     paths:
       - ".github/workflows/platform-frontend-ci.yml"
       - "autogpt_platform/frontend/**"
+      - "autogpt_platform/backend/Dockerfile"
   pull_request:
     paths:
       - ".github/workflows/platform-frontend-ci.yml"
       - "autogpt_platform/frontend/**"
+      - "autogpt_platform/backend/Dockerfile"
   merge_group:
   workflow_dispatch: