Add Docker-based Linux release build for Ubuntu 20.04 compatibility

Dockerfile.build

@@ -0,0 +1,17 @@
+FROM ubuntu:20.04
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update && apt-get install -y \
+    build-essential cmake pkg-config libtool autoconf automake \
+    curl git m4 g++ gcc libcurl4-gnutls-dev zlib1g-dev \
+    libsodium-dev unzip zip bsdmainutils \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /hush3
+
+# Build script - runs inside container
+COPY build-docker-release.sh /build-docker-release.sh
+RUN chmod +x /build-docker-release.sh
+
+ENTRYPOINT ["/build-docker-release.sh"]