Add --linux-compat build option for Ubuntu 20.04 binaries

Build release binaries inside an Ubuntu 20.04 Docker container
to produce executables with lower GLIBC requirements, compatible
with older Linux distributions.

- Add Dockerfile.compat (Ubuntu 20.04 base, full depends rebuild)
- Add .dockerignore to exclude host build artifacts from context
- Add --linux-compat flag to build.sh with Docker build/extract/package
- Strip binaries inside container to avoid root ownership issues

build.sh

@@ -14,6 +14,7 @@ RELEASE_DIR="$SCRIPT_DIR/release"
 BUILD_LINUX_RELEASE=0
 BUILD_WIN_RELEASE=0
 BUILD_MAC_RELEASE=0
+BUILD_LINUX_COMPAT=0
 REMAINING_ARGS=()
 
 for arg in "$@"; do
@@ -21,6 +22,9 @@ for arg in "$@"; do
         --linux-release)
             BUILD_LINUX_RELEASE=1
             ;;
+        --linux-compat)
+            BUILD_LINUX_COMPAT=1
+            ;;
         --win-release)
             BUILD_WIN_RELEASE=1
             ;;
@@ -110,8 +114,59 @@ package_release() {
 }
 
 # Handle release builds
-if [ $BUILD_LINUX_RELEASE -eq 1 ] || [ $BUILD_WIN_RELEASE -eq 1 ] || [ $BUILD_MAC_RELEASE -eq 1 ]; then
+if [ $BUILD_LINUX_COMPAT -eq 1 ] || [ $BUILD_LINUX_RELEASE -eq 1 ] || [ $BUILD_WIN_RELEASE -eq 1 ] || [ $BUILD_MAC_RELEASE -eq 1 ]; then
     mkdir -p "$RELEASE_DIR"
+
+    if [ $BUILD_LINUX_COMPAT -eq 1 ]; then
+        echo "=== Building Linux compat release (Ubuntu 20.04 via Docker) ==="
+        if ! command -v docker &>/dev/null; then
+            echo "Error: docker is required for --linux-compat builds"
+            exit 1
+        fi
+
+        # Use sudo for docker if the user isn't in the docker group
+        DOCKER_CMD="docker"
+        if ! docker info &>/dev/null 2>&1; then
+            echo "Note: Using sudo for docker (add yourself to the docker group to avoid this)"
+            DOCKER_CMD="sudo docker"
+        fi
+
+        DOCKER_IMAGE="dragonx-compat-builder"
+        COMPAT_PLATFORM="linux-amd64-ubuntu2004"
+        COMPAT_RELEASE_DIR="$RELEASE_DIR/dragonx-$VERSION-$COMPAT_PLATFORM"
+
+        echo "Building Docker image (Ubuntu 20.04 base)..."
+        $DOCKER_CMD build -f Dockerfile.compat -t "$DOCKER_IMAGE" .
+
+        echo "Extracting binaries from Docker image..."
+        CONTAINER_ID=$($DOCKER_CMD create "$DOCKER_IMAGE")
+        mkdir -p "$COMPAT_RELEASE_DIR"
+
+        for bin in dragonxd dragonx-cli dragonx-tx; do
+            $DOCKER_CMD cp "$CONTAINER_ID:/build/src/$bin" "$COMPAT_RELEASE_DIR/$bin"
+        done
+        $DOCKER_CMD rm "$CONTAINER_ID" >/dev/null
+
+        # Fix ownership (docker cp creates root-owned files)
+        # Binaries are already stripped inside the Docker container
+        if [ "$(stat -c '%U' "$COMPAT_RELEASE_DIR/dragonxd")" = "root" ]; then
+            sudo chown "$(id -u):$(id -g)" "$COMPAT_RELEASE_DIR"/dragonx*
+        fi
+
+        # Copy common files
+        cp "$SCRIPT_DIR/util/bootstrap-dragonx.sh" "$COMPAT_RELEASE_DIR/"
+        cp "$SCRIPT_DIR/contrib/asmap/asmap.dat" "$COMPAT_RELEASE_DIR/" 2>/dev/null || true
+        cp "$SCRIPT_DIR/sapling-output.params" "$COMPAT_RELEASE_DIR/" 2>/dev/null || true
+        cp "$SCRIPT_DIR/sapling-spend.params" "$COMPAT_RELEASE_DIR/" 2>/dev/null || true
+
+        echo "Compat release packaged: $COMPAT_RELEASE_DIR"
+        ls -la "$COMPAT_RELEASE_DIR"
+
+        # Show glibc version requirement
+        echo ""
+        echo "Binary compatibility info:"
+        objdump -T "$COMPAT_RELEASE_DIR/dragonxd" | grep -oP 'GLIBC_\d+\.\d+' | sort -uV | tail -1 && echo "(max GLIBC version required)"
+    fi
     
     if [ $BUILD_LINUX_RELEASE -eq 1 ]; then
         echo "=== Building Linux release ==="