Add a Docker-based build image and wire it into CI for the watch app

README.md

@@ -60,6 +60,10 @@ watch/                            The actual Pebble watchapp - links engine/src/
                                   from the rest of this repo (`make watchapp`,
                                   below) - requires the Pebble SDK.
 docs/                            Architecture/dataflow diagrams, input/output format reference.
+build-image/                     Dockerfile for the reproducible build environment
+                                  (C toolchain + Pebble SDK) used by CI and by the
+                                  build-image.sh/run-image.sh/upload-image.sh scripts
+                                  at the repo root (see "Building" below).
 Makefile                         Single root Makefile, builds both engine/ and
                                   interpreter/, and packages a release (`make package`).
 ```
@@ -94,6 +98,20 @@ make watchapp # -> dist/deck-in-a-dash-<version>.pbw, same version
               #    resolution as `make package`
 ```
 
+**CI runs `make watchapp` too** — `.gitea/workflows/build.yml` builds
+and tests everything inside a container built from `build-image/`,
+which bundles the Pebble SDK alongside the C toolchain so the runner
+itself doesn't need either installed. To build/test that image
+yourself, copy `registry.env.example` to `registry.env` and fill in
+your registry credentials, then:
+
+```bash
+./build-image.sh   # build and locally tag the image
+./run-image.sh     # run `make test package watchapp` inside it, against
+                    # this checkout, for a CI-equivalent local build
+./upload-image.sh  # push the image so CI can pull it
+```
+
 ## Running
 
 **Daily use**, via `dist/run-engine.sh`: pulls today's date and the