docker: update web image build process and README

- Added a new target "web-build" to streamline the build process for web artifacts.
- Updated "evobgp-web" and "evobgp-web-all" targets to utilize the new "web-build" context.
- Modified Dockerfile to reflect changes in the build stages and context for web artifacts.
- Revised README to clarify the new build process and dependencies.

deploy/docker/evobgp-web/Dockerfile

@@ -1,5 +1,6 @@
 # syntax=docker/dockerfile:1.7
 # Статическая панель EvoBGP (SvelteKit) + nginx.
+# Финальный stage `web` ожидает bake-контекст web-artifacts (= target:web-build).
 FROM public.ecr.aws/docker/library/node:22-alpine AS deps
 WORKDIR /web
 COPY web/package.json web/package-lock.json ./
@@ -10,9 +11,9 @@ FROM deps AS build
 COPY web/ ./
 RUN npm run build
 
-FROM public.ecr.aws/docker/library/nginx:1.27-alpine
+FROM public.ecr.aws/docker/library/nginx:1.27-alpine AS web
 ARG EVOBGP_UPSTREAM=evobgp-api
 COPY deploy/docker/evobgp-web/nginx.conf /tmp/nginx-default.conf
 RUN sed -e "s/evobgp-api/${EVOBGP_UPSTREAM}/g" /tmp/nginx-default.conf > /etc/nginx/conf.d/default.conf \
     && rm -f /tmp/nginx-default.conf
-COPY --from=build /web/build /usr/share/nginx/html
+COPY --from=web-artifacts /web/build /usr/share/nginx/html