refactor: appiattisce docker/build/ in docker/

Con la rimozione di docker/dev/ non serve più distinguere due
container: Dockerfile e build.sh salgono di un livello, con i path
relativi (PROJECT_ROOT, keystore) aggiornati di conseguenza.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docker/Dockerfile

@@ -0,0 +1,36 @@
+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:/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 ${FLUTTER_VERSION} \
+        https://github.com/flutter/flutter.git ${FLUTTER_HOME} && \
+    flutter config --no-analytics && \
+    flutter precache --android
+
+RUN mkdir -p ${ANDROID_SDK_ROOT}/cmdline-tools && \
+    wget -q "https://dl.google.com/android/repository/commandlinetools-linux-${ANDROID_SDK_BUILD}_latest.zip" \
+         -O /tmp/cmdtools.zip && \
+    unzip -q /tmp/cmdtools.zip -d ${ANDROID_SDK_ROOT}/cmdline-tools && \
+    mv ${ANDROID_SDK_ROOT}/cmdline-tools/cmdline-tools \
+       ${ANDROID_SDK_ROOT}/cmdline-tools/latest && \
+    rm /tmp/cmdtools.zip
+
+RUN yes | sdkmanager --licenses && \
+    sdkmanager \
+        "platform-tools" \
+        "platforms;android-34" \
+        "build-tools;34.0.0"
+
+WORKDIR /workspace