ci: build base image on every pull request

[TimPietruskyRunPod]

Summary

Today the `Development` workflow only runs on manual `workflow_dispatch`, and the `release` workflow only builds images after a `chore: version packages` commit lands on `main`. So PR changes can't be deployed to a real serverless endpoint until they're already merged. That makes it impossible to validate behavior before approval — we're flying blind.

This PR makes `Development` build the `base` image on every PR push (opened / synchronize / reopened / ready_for_review), tagged with the branch slug. So opening this very PR should produce `runpod/worker-comfyui:ci-build-on-pull-request-base` (or whatever the configured DOCKERHUB_REPO/IMG resolves to). Then deploying a serverless endpoint with that image lets us test the PR.

Changes

• `.github/workflows/dev.yml`
  • Add `pull_request` trigger with `paths-ignore` for docs/changeset-only PRs
  • Compute `RELEASE_VERSION` from `GITHUB_HEAD_REF` first (PR source branch), falling back to `GITHUB_REF` for push/dispatch
  • Slug branch name into Docker-safe tag (replace anything not `[a-zA-Z0-9._-]` with `-`)

Why `paths-ignore`

Builds are heavy (~20-30 min on Blacksmith). Skip the build for PRs that only touch markdown, `docs/`, or `.changeset/` files. Behavior-affecting PRs (anything in `src/`, `handler.py`, `Dockerfile`, workflows, `.runpod/`) still trigger a build.

Risks

• Every PR now consumes Blacksmith runner minutes for a `base` build. If that becomes too expensive, we can switch to label-gated builds (only build if a `build` label is present).
• Docker Hub will accumulate one tag per branch. `docker-prune.yml` already exists to clean these up periodically — recommend extending it if needed.

Test plan

• Push to this branch and confirm `Development` workflow triggers
• Confirm the produced image tag matches the branch slug
• Deploy a serverless endpoint from the new image and submit a real job to confirm the worker still functions normally (no behavior change, just CI plumbing)

[TimPietruskyRunPod]

Validation status: this PR's own workflow works ✅

Used `gh workflow run dev.yml --ref ci/build-on-pull-request` (workflow_dispatch on this branch) which exercises the new trigger / tag logic from this PR. Build succeeded in ~5 min and the image was pushed as expected. Then dispatched the same way on #224 and #225 — both succeeded, tagging cleanly as `runpod/worker-comfyui:fix-pin-boto3-for-non-aws-s3-base` and `runpod/worker-comfyui:fix-enable-png-workflow-metadata-base` respectively.

What I didn't directly validate: that the `pull_request` trigger fires automatically when a PR is opened. That can only be tested by actually opening the next PR after this lands and watching for the workflow. Low risk — the GitHub Actions `pull_request` trigger is well-trodden ground.

Recommend merging. Once merged, all subsequent PR images get built automatically and we can validate #224/#225/#191/#221/#222 etc. on real endpoints without manual dispatches.