Wait for docker pull in CI (#2023)

CI can fail if `docker pull ...` takes longer than expected. This PR
adds a `--pull-only` option to `./start`, then waits for it to complete
in CI.

.github/workflows/main.yml

@@ -51,11 +51,14 @@ jobs:
         with:
           files: docs/**/*.{md,mdx,ipynb}
           separator: "\n"
+      - name: Pull preview image
+        if: steps.changed-docs-files.outputs.any_changed == 'true'
+        run: ./start --pull-only
       - name: Start local Docker preview
         if: steps.changed-docs-files.outputs.any_changed == 'true'
         run: |
           ./start --apis &
-          sleep 20
+          sleep 1
       - name: Check that pages render
         if: steps.changed-docs-files.outputs.any_changed == 'true'
         run: |

start

@@ -27,6 +27,10 @@ def skip_apis() -> tuple[str]:
     return ("-v", "/home/node/app/docs/api")
 
 def main() -> None:
+    if "--pull-only" in sys.argv:
+        subprocess.run(["docker", "pull", IMAGE], check=True)
+        sys.exit(0)
+
     print(
         "Warning: this may be using an outdated version of the app. Run "
         + f"`docker pull {IMAGE}` to check for updates.",