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/README.md

@@ -8,7 +8,7 @@
 |------|--------|
 | 8× `go mod download` + 8× `go build` (разные BIN) | 1× download + 1× компиляция всех `cmd/*` |
 | 2× сборка BIRD из исходников (api, all) | 1× stage `birdc`, копируется в runtime |
-| 2× `npm ci` (web, web-all) | 1× `web-deps` + два nginx-образа |
+| 2× `npm ci` (web, web-all) | 1× `web-deps` + 1× `web-build` + два nginx-образа (общий артефакт) |
 | 8 runner'ов с checkout/login | 1 job `docker-go`, 1 job `docker-web` |
 
 Кэш registry (переменные bake):