docs: clarify "primary docker image" error is an LLM backend failure

[mason5052]

Summary

Add a troubleshooting subsection to the README "Docker Image Configuration" section clarifying that the failed to select primary docker image via llm call error (older versions: failed to get primary docker image) is an LLM backend failure during image selection, not a Docker or registry problem.

Refs #309

Problem

On a fresh install, creating a flow can fail immediately with:

failed to get flow provider: failed to select primary docker image via llm call: API returned unexpected status code: 502

The phrase "primary docker image" leads users to debug Docker, the image registry, or pull access even though Docker is healthy. As the maintainer noted on #309 (and on the related #312 and #203), the real cause is the configured LLM backend: PentAGI makes its first LLM call to choose the primary image when a flow starts, so a backend failure (502/404, unreachable endpoint, wrong model, or a missing/mismatched tool-call parser on custom/sglang-style backends) surfaces at this step.

The error string itself was already made more precise in #312 / #320 (failed to get primary docker image -> failed to select primary docker image via llm call). This PR closes the remaining documentation gap so users can act on that clarified message.

Solution

Add a short #### Troubleshooting subsection under "Docker Image Configuration" that:

• States the error indicates an LLM backend problem, not Docker or the registry.
• Explains the image-selection-at-flow-start mechanism so the origin of a 502/404 is clear.
• Distinguishes it from the existing "Restricted Networks, Docker Mirrors, and Proxies" subsection, which covers genuine registry reachability.
• Gives concrete diagnosis steps: check docker logs pentagi, check the LLM backend logs, verify the provider URL/key/model, and confirm tool-call / function-calling parser support for custom, OpenAI-compatible, vLLM, or sglang backends.
• Cross-links to the existing "Custom LLM Provider Configuration" section.

Documentation only. No runtime, schema, config, or behavior changes.

User Impact

Users who hit this error can immediately look in the right place (the LLM backend) instead of debugging Docker, reducing time-to-resolution for a recurring fresh-install confusion reported across #309, #312, and #203.

Test Plan

• Documentation-only change; no Go or frontend code touched.
• git diff --check reports no whitespace errors.
• Verified only README.md is modified (15 insertions).
• Verified the cross-link anchor #custom-llm-provider-configuration resolves to the existing "Custom LLM Provider Configuration" heading.
• Reviewed the rendered Markdown for correct heading, list, and code-span formatting.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds targeted documentation to help users distinguish LLM-backend failures from Docker/registry issues when flow creation fails during primary image selection.

Changes:

• Documented what the "failed to select primary docker image via llm call" error usually indicates.
• Added diagnostic steps focusing on PentAGI and LLM backend configuration/logs.
• Clarified common misdirection from older PentAGI error wording.

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