ci: pass --target build-env to docker build in APK workflow

After the Dockerfile refactor into multi-stage (commit 6f975f87),
'docker build .' without --target selects the LAST stage of the
Dockerfile — which is blogs-runtime, an ASP.NET image with no
APK to extract. The subsequent docker cp command then fails
with:

  Error: No such container:path: …PostIt.Android/bin/Release/
  net10.0-android/android-arm64/com.CompanyName.PostIt-Signed.apk

Add --target build-env to scope the build to the build-env stage
(the one that produces both the .apk and the publish artifacts,
and which still ends with CMD ["bash"]).

.github/workflows/docker-publish-android.yml

@@ -14,8 +14,12 @@ jobs:
         uses: actions/checkout@v7
 
       # 1. Votre étape de build actuelle (on nomme l'image "postit-android")
+      #    --target build-env : on ne veut que le stage de build (qui
+      #    contient les artefacts .apk). Sans --target, Docker ciblerait
+      #    le DERNIER stage du Dockerfile (blogs-runtime, qui est une
+      #    image ASP.NET runtime sans aucun APK à extraire).
       - name: Build de l'image Docker
-        run: docker build --build-arg ANDROID_TARGET_RID=android-arm64 -t postit-android .
+        run: docker build --build-arg ANDROID_TARGET_RID=android-arm64 --target build-env -t postit-android .
       # 2. EXTRACTION : Créer un conteneur éphémère pour copier l'APK vers l'hôte GitHub
       - name: Extraire l'APK du conteneur Docker
         run: |