I try to build an APK of my Flutter application with Docker.

At the last step I have this error:

> [17/17] RUN flutter build apk --release:
1.420 
1.496 Running Gradle task 'assembleRelease'...                        
12.76 Exception in thread "main" java.net.UnknownHostException: objects.githubusercontent.com
12.76   at java.base/sun.nio.ch.NioSocketImpl.connect(NioSocketImpl.java:572)
12.76   at java.base/java.net.SocksSocketImpl.connect(SocksSocketImpl.java:327)
12.76   at java.base/java.net.Socket.connect(Socket.java:633)
12.76   at java.base/sun.security.ssl.SSLSocketImpl.connect(SSLSocketImpl.java:304)
12.76   at java.base/sun.security.ssl.BaseSSLSocketImpl.connect(BaseSSLSocketImpl.java:174)
12.76   at java.base/sun.net.NetworkClient.doConnect(NetworkClient.java:183)
12.76   at java.base/sun.net.www.http.HttpClient.openServer(HttpClient.java:533)
12.76   at java.base/sun.net.www.http.HttpClient.openServer(HttpClient.java:638)
12.76   at java.base/sun.net.www.protocol.https.HttpsClient.<init>(HttpsClient.java:266)
12.76   at java.base/sun.net.www.protocol.https.HttpsClient.New(HttpsClient.java:380)
12.76   at java.base/sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.getNewHttpClient(AbstractDelegateHttpsURLConnection.java:193)
12.76   at java.base/sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1241)
12.76   at java.base/sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1127)
12.76   at java.base/sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:179)
12.76   at java.base/sun.net.www.protocol.http.HttpURLConnection.followRedirect0(HttpURLConnection.java:2943)
12.76   at java.base/sun.net.www.protocol.http.HttpURLConnection.followRedirect(HttpURLConnection.java:2852)
12.76   at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1948)
12.76   at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1610)
12.76   at java.base/sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:224)
12.76   at org.gradle.wrapper.Download.downloadInternal(Download.java:58)
12.76   at org.gradle.wrapper.Download.download(Download.java:44)
12.76   at org.gradle.wrapper.Install$1.call(Install.java:61)
12.76   at org.gradle.wrapper.Install$1.call(Install.java:48)
12.76   at org.gradle.wrapper.ExclusiveFileAccessManager.access(ExclusiveFileAccessManager.java:65)
12.76   at org.gradle.wrapper.Install.createDist(Install.java:48)
12.77   at org.gradle.wrapper.WrapperExecutor.execute(WrapperExecutor.java:128)
12.77   at org.gradle.wrapper.GradleWrapperMain.main(GradleWrapperMain.java:61)
12.77 Running Gradle task 'assembleRelease'...                           11.3s
12.83 Gradle task assembleRelease failed with exit code 1`

Here is my Dockerfile:

# Use a base image with necessary tools for building Flutter apps
FROM ubuntu:22.04

# Install required dependencies for Flutter, Android, and Linux development
RUN apt-get update && apt-get install -y 
    git 
    wget 
    unzip 
    curl 
    cmake 
    clang 
    ninja-build 
    pkg-config 
    libgtk-3-dev 
    liblzma-dev 
    openjdk-17-jdk 
    build-essential 
    libgl1-mesa-dev 
    && rm -rf /var/lib/apt/lists/*

# Install Android SDK and command-line tools
RUN mkdir -p /opt/android-sdk-linux/cmdline-tools/latest && 
    cd /opt/android-sdk-linux/cmdline-tools/latest && 
    wget https://dl.google.com/android/repository/commandlinetools-linux-8512546_latest.zip && 
    unzip commandlinetools-linux-8512546_latest.zip && 
    mv cmdline-tools/* . && 
    rm -rf cmdline-tools commandlinetools-linux-8512546_latest.zip

# Set environment variables for Android SDK and Flutter
ENV ANDROID_HOME=/opt/android-sdk-linux
ENV PATH=$ANDROID_HOME/cmdline-tools/latest/bin:$ANDROID_HOME/platform-tools:$PATH

# Ensure sdkmanager is executable
RUN chmod +x /opt/android-sdk-linux/cmdline-tools/latest/bin/sdkmanager

RUN sdkmanager --version

# Accept licenses and install SDK components using sdkmanager
RUN yes | sdkmanager --licenses && 
    sdkmanager "platform-tools" "platforms;android-33" "build-tools;33.0.0"

# Create the non-root user before cloning Flutter
RUN useradd -m -s /bin/bash flutteruser

# Create /opt/flutter directory and change ownership to the non-root user
RUN mkdir -p /opt/flutter && chown -R flutteruser:flutteruser /opt/flutter /opt/android-sdk-linux

# Switch to the non-root user and clone Flutter into /opt/flutter
USER flutteruser
RUN git clone https://github.com/flutter/flutter.git /opt/flutter

# Set Flutter in the PATH for non-root user
ENV PATH="/opt/flutter/bin:$ANDROID_HOME/cmdline-tools/latest/bin:$ANDROID_HOME/platform-tools:$PATH"

# Set working directory
WORKDIR /app

# Set the Flutter channel to stable and upgrade
RUN flutter channel stable && flutter upgrade

# Accept Android SDK licenses
RUN yes | flutter doctor --android-licenses

# Switch back to root to copy project files and change their ownership
USER root
# Copy the Flutter project files into the container
COPY . .

# Change ownership of the project files to flutteruser
RUN chown -R flutteruser:flutteruser /app

# Switch back to the non-root user to run the rest of the commands
USER flutteruser

# Install dependencies
RUN flutter pub get

# Check the status of the Flutter installation
RUN flutter doctor -v

# Build the APK in release mode
RUN flutter build apk --release

# The APK will be located in /app/build/app/outputs/flutter-apk/

I run my Dockerfile with the command:
docker build -t my-flutter-app .

I try also with --network=host but I have the same error.

I think it might be a Network error but I have no idea how to solve it…

Thank you by advance for your help.