Split the monolithic build-image.sh into three scripts: build-image.sh now only builds locally, run-image.sh

runs dist.sh inside that local image via a bind mount, andcupload-image.sh pushes the already-built image.
The Dockerfile also warms the Gradle dependency cache in a throwaway build stage, and pre-installs
the pinned CMake version, so `run-image.sh` doesn't re-download the same Maven dependencies on every run.

dist.sh now strips the -pre+meta suffix before writing package.json's version, since Pebble's build
tooling parses it strictly as X.Y.Z integers and was rejecting suffixed versions.

upload-image.sh

@@ -0,0 +1,41 @@
+#!/usr/bin/env bash
+# Pushes the build environment image — already built locally with
+# ./build-image.sh, and ideally verified with ./run-image.sh — to
+# cr.ladkau.de. The Gitea Actions release workflow pulls this image to
+# run dist.sh.
+set -euo pipefail
+
+cd "$(dirname "${BASH_SOURCE[0]}")"
+ROOT="$(pwd)"
+
+fail() { echo "PREFLIGHT FAIL: $*" >&2; exit 1; }
+
+command -v docker >/dev/null 2>&1 \
+    || fail "docker not found in PATH"
+
+[ -f "$ROOT/registry.env" ] \
+    || fail "registry.env not found — copy registry.env.example to registry.env and fill in your cr.ladkau.de credentials"
+
+# shellcheck disable=SC1091
+source "$ROOT/registry.env"
+
+: "${REGISTRY:?registry.env must set REGISTRY}"
+: "${REGISTRY_IMAGE:?registry.env must set REGISTRY_IMAGE}"
+: "${REGISTRY_USER:?registry.env must set REGISTRY_USER}"
+: "${REGISTRY_PASSWORD:?registry.env must set REGISTRY_PASSWORD}"
+
+VERSION="$(<"$ROOT/build-image/VERSION")"
+[ -n "$VERSION" ] || fail "build-image/VERSION is empty"
+
+docker image inspect "$REGISTRY_IMAGE:$VERSION" >/dev/null 2>&1 \
+    || fail "$REGISTRY_IMAGE:$VERSION not found locally — run ./build-image.sh first"
+
+echo "== Logging in to $REGISTRY =="
+echo "$REGISTRY_PASSWORD" | docker login "$REGISTRY" -u "$REGISTRY_USER" --password-stdin
+
+echo "== Pushing $REGISTRY_IMAGE:$VERSION and :latest =="
+docker push "$REGISTRY_IMAGE:$VERSION"
+docker push "$REGISTRY_IMAGE:latest"
+
+echo "== Done =="
+echo "Image: $REGISTRY_IMAGE:$VERSION"