#9102: Remove UI build for Discord-onlyUpdate Dockerfile

Remove UI build for Discord-only <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates the `Dockerfile` to remove the explicit `pnpm ui:build` layer, leaving the container build to run only `OPENCLAW_A2UI_SKIP_MISSING=1 pnpm build` before switching to a non-root `node` user and starting the gateway server. This fits with a Discord-only deployment where the UI assets are not needed, reducing build work and avoiding UI build failures on some architectures. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - Change is limited to removing a single Docker build step; it does not affect runtime command, permissions model, or application code paths. No indications of broken build dependencies were introduced in the Dockerfile itself. - No files require special attention <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->