build(docker): add armv7l cross-compilation support

- Add Dockerfile.linux-armv7l for ARMv7l cross-compilation environment
- Create build-linux-armv7l.sh script to automate the build process
- Update .dockerignore to exclude more build artifacts and temporary files
- Extend README.md with ARMv7l build instructions and target details

docker-build/Dockerfile.linux-armv7l

@@ -0,0 +1,16 @@
+FROM ubuntu:20.04
+
+ENV DEBIAN_FRONTEND=noninteractive TZ=UTC
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    build-essential libtool autotools-dev automake autoconf pkg-config \
+    bsdmainutils python3 curl ca-certificates git unzip zip file rsync \
+    make cmake patch \
+    g++-arm-linux-gnueabihf binutils-arm-linux-gnueabihf \
+ && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /src
+
+COPY . /src
+
+CMD ["/bin/bash"]