observer-web: multi-stage Dockerfile to reduce registry push size.

Co-authored-by: Cursor <cursoragent@cursor.com>

docker/ObserverWeb.Dockerfile

@@ -1,7 +1,7 @@
-FROM erlang:28-alpine
+FROM erlang:28-alpine AS builder
 
 RUN for i in 1 2 3 4 5; do \
-      apk add --no-cache elixir nodejs npm git inotify-tools && break; \
+      apk add --no-cache elixir nodejs npm git && break; \
       echo "apk retry $i/5..."; sleep 15; \
     done
 
@@ -20,9 +20,19 @@ RUN mkdir -p priv/static && \
     npm install && \
     npm install --save-dev tailwindcss@4.1.0 @tailwindcss/cli @tailwindcss/forms esbuild@0.17.11 && \
     cd .. && \
-    mix assets.build
+    mix assets.build && \
+    rm -rf assets/node_modules /root/.npm /root/.cache .git
 
-COPY docker/observer_web/dev.exs .
+FROM erlang:28-alpine AS runtime
+
+RUN for i in 1 2 3 4 5; do \
+      apk add --no-cache elixir inotify-tools && break; \
+      echo "apk retry $i/5..."; sleep 15; \
+    done
+
+WORKDIR /app
+COPY --from=builder /app /app
+COPY docker/observer_web/dev.exs ./dev.exs
 
 EXPOSE 4000