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

Dockerfile.compat

@@ -0,0 +1,31 @@
+FROM ubuntu:20.04
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update && apt-get install -y \
+    build-essential pkg-config libc6-dev m4 g++-multilib autoconf libtool \
+    ncurses-dev unzip python3 zlib1g-dev wget bsdmainutils automake cmake \
+    libcurl4-openssl-dev curl git binutils \
+    && apt-get clean && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /build
+COPY . /build/
+
+# Clean host-built depends and src artifacts to force full rebuild inside container
+RUN rm -rf /build/depends/built /build/depends/work \
+    /build/depends/x86_64-unknown-linux-gnu \
+    /build/depends/x86_64-w64-mingw32 \
+    /build/src/RandomX/build \
+    && find /build/src -name '*.o' -o -name '*.a' -o -name '*.la' -o -name '*.lo' \
+       -o -name '*.lai' | xargs rm -f \
+    && rm -rf /build/src/univalue/.libs /build/src/univalue/.deps \
+    && rm -rf /build/src/.libs /build/src/.deps \
+    && rm -rf /build/src/cc/*.o /build/src/cc/*.a \
+    && rm -f /build/config.status /build/config.log
+
+RUN cd /build && ./util/build.sh --disable-tests -j$(nproc)
+
+# Strip binaries inside the container so extracted files are already small
+RUN strip /build/src/dragonxd /build/src/dragonx-cli /build/src/dragonx-tx
+
+CMD ["/bin/bash"]