build: rendi la build Docker riproducibile

- Pinna Flutter a 3.41.9 (ARG FLUTTER_VERSION) in entrambi i Dockerfile
- Aggiunge /opt/android-sdk/build-tools/34.0.0 al PATH (zipalign era missing)
- Sostituisce mount ${HOME}/.gradle con named volume gradle-cache
- Aggiunge --enforce-lockfile a flutter pub get per bloccare le dipendenze
- Aggiunge .flutter-version con la versione Flutter attesa

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker/build/Dockerfile

@@ -1,19 +1,20 @@
 FROM ubuntu:22.04
 
+ARG FLUTTER_VERSION=3.41.9
 ARG ANDROID_SDK_BUILD=11076708
 
 ENV DEBIAN_FRONTEND=noninteractive \
     FLUTTER_HOME=/opt/flutter \
     ANDROID_SDK_ROOT=/opt/android-sdk \
     JAVA_HOME=/usr/lib/jvm/java-17-openjdk-amd64 \
-    PATH="/opt/flutter/bin:/opt/android-sdk/cmdline-tools/latest/bin:/opt/android-sdk/platform-tools:${PATH}"
+    PATH="/opt/flutter/bin:/opt/android-sdk/cmdline-tools/latest/bin:/opt/android-sdk/platform-tools:/opt/android-sdk/build-tools/34.0.0:${PATH}"
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
     curl wget git unzip xz-utils ca-certificates \
     openjdk-17-jdk \
     && rm -rf /var/lib/apt/lists/*
 
-RUN git clone --depth 1 --branch stable \
+RUN git clone --depth 1 --branch ${FLUTTER_VERSION} \
         https://github.com/flutter/flutter.git ${FLUTTER_HOME} && \
     flutter config --no-analytics && \
     flutter precache --android

docker/dev/Dockerfile

@@ -1,5 +1,7 @@
 FROM ubuntu:22.04
 
+ARG FLUTTER_VERSION=3.41.9
+
 ENV DEBIAN_FRONTEND=noninteractive \
     FLUTTER_HOME=/opt/flutter \
     PATH="/opt/flutter/bin:${PATH}"
@@ -8,7 +10,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     curl git unzip xz-utils ca-certificates \
     && rm -rf /var/lib/apt/lists/*
 
-RUN git clone --depth 1 --branch stable \
+RUN git clone --depth 1 --branch ${FLUTTER_VERSION} \
         https://github.com/flutter/flutter.git /opt/flutter && \
     flutter config --no-analytics --enable-web && \
     flutter precache --web