♻️(ci) reuse amd64 to build arm64 image when possible

Building twice the image take lof of time. In soma cases, building the
arm64 image using the artifacts build in the amd64 and thant can be
reused should speed up the build of the arm64 image.

src/frontend/Dockerfile

@@ -1,3 +1,5 @@
+ARG FRONTEND_IMAGE=frontend-build-output
+
 FROM node:24-alpine AS frontend-deps
 
 # Upgrade system packages to install security updates
@@ -32,7 +34,7 @@ EXPOSE 3000
 
 CMD [ "yarn", "dev"]
 
-# Tilt will rebuild impress target so, we dissociate impress and impress-builder 
+# Tilt will rebuild impress target so, we dissociate impress and impress-builder
 # to avoid rebuilding the app at every changes.
 FROM impress AS impress-builder
 
@@ -49,6 +51,14 @@ ENV NEXT_PUBLIC_PUBLISH_AS_MIT=${PUBLISH_AS_MIT}
 
 RUN yarn build
 
+# Normalize output path to /app (matches the runtime-prod layout)
+FROM scratch AS frontend-build-output
+COPY --from=impress-builder /home/frontend/apps/impress/out /app
+
+# When FRONTEND_IMAGE is set to an external image, BuildKit skips
+# frontend-deps,impress-builder, and frontend-build-output entirely
+FROM ${FRONTEND_IMAGE} AS frontend-source
+
 # ---- Front-end image ----
 FROM nginxinc/nginx-unprivileged:alpine3.22 AS frontend-production
 
@@ -62,9 +72,7 @@ RUN apk update && \
 ARG DOCKER_USER
 USER ${DOCKER_USER}
 
-COPY --from=impress-builder \
-    /home/frontend/apps/impress/out \
-    /usr/share/nginx/html
+COPY --from=frontend-source /app /app
 
 COPY ./src/frontend/apps/impress/conf/default.conf /etc/nginx/conf.d
 COPY ./docker/files/usr/local/bin/entrypoint /usr/local/bin/entrypoint

src/frontend/apps/impress/conf/default.conf

@@ -3,7 +3,7 @@ server {
   listen 3000;
   server_name localhost;
 
-  root /usr/share/nginx/html;
+  root /app;
 
   location / {
       try_files $uri index.html $uri/index.html =404;