docs(BestPractices): move the warning into the node-gyp section, as reviewed

Replaces the added 'Multi-architecture images' heading (and its doctoc entry) with a short
note under the multistage example that already demonstrates the pattern - COPY --from=builder
node_modules. Six lines instead of twenty, no new section to maintain.

Validated with this repo's own checks: prettier --check, doctoc --update-only --dryrun.

Author: happy520ai

docs/BestPractices.md

@@ -15,7 +15,6 @@
 - [Docker Run](#docker-run)
 - [Security](#security)
 - [node-gyp alpine](#node-gyp-alpine)
-- [Multi-architecture images](#multi-architecture-images)
 - [Smaller images without npm/yarn](#smaller-images-without-npmyarn)
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
@@ -181,24 +180,12 @@ FROM node:alpine as app
 COPY --from=builder node_modules .
 ```
 
-## Multi-architecture images
-
-A dependency tree installed once cannot serve two architectures. `docker buildx build` with
-`--platform linux/amd64,linux/arm64` builds each stage natively per platform, but if one stage runs
-`npm ci` on the build host and a later stage copies the result with
-`COPY --from=builder node_modules .`, that same tree - carrying the build host's prebuilt native
-addons - ends up inside every platform tag. The manifest still advertises both architectures, so nothing
-looks wrong until an arm64 host loads an x86-64 binary:
-
-```console
-Error: /app/node_modules/better-sqlite3/build/Release/better_sqlite3.node: invalid ELF header
-```
-
-Install dependencies inside each platform's stage (or rebuild them there with `npm rebuild`), and
-publish only the architectures actually built. To check an image that is already published, without a
-Docker engine: fetch its manifest from the registry, decompress the layer tarballs, and read the
-`e_machine` field of any `.node` file - two bytes at offset 18, `0x3e` for x86-64 and `0xb7` for
-AArch64.
+Note that this multistage pattern only produces a tree that works on the architecture the builder ran on.
+Native addons are compiled or downloaded for that host, so `COPY --from=builder node_modules .` into a stage
+built for a different architecture puts a mismatched binary in the image while the manifest still looks
+correct, and the first `require()` of it fails with `invalid ELF header`. Under
+`docker buildx build --platform linux/amd64,linux/arm64`, install or `npm rebuild` inside each platform's
+stage instead of sharing one tree between them.
 
 ## Smaller images without npm/yarn