feat(build): add support for armv7

docker/cross-arm.Dockerfile

+FROM rust:latest
+
+RUN dpkg --add-architecture armhf && \
+  apt-get update && apt-get upgrade -y && \
+  apt-get install -y aptitude && \
+  aptitude install -y \
+  gcc-arm-linux-gnueabihf \
+  g++-arm-linux-gnueabihf \
+  pkg-config \
+  cmake \
+  libssl-dev \
+  git \
+  clang \
+  libclang-dev \
+  libssl-dev:armhf
+
+# Install nightly with armv7 and  w32-u-u
+RUN rustup install nightly-2021-11-12-x86_64-unknown-linux-gnu && \
+  rustup target add armv7-unknown-linux-gnueabihf --toolchain \
+  nightly-2021-11-12-x86_64-unknown-linux-gnu && \
+  rustup target add wasm32-unknown-unknown --toolchain \
+  nightly-2021-11-12-x86_64-unknown-linux-gnu && \
+  cargo +nightly-2021-11-12-x86_64-unknown-linux-gnu install --git \
+  https://github.com/alexcrichton/wasm-gc --force
+
+ENV CARGO_TARGET_ARMV7_UNKNOWN_LINUX_GNUEABIHF_LINKER arm-linux-gnueabihf-gcc
+ENV PKG_CONFIG_ALLOW_CROSS 1
+ENV PKG_CONFIG_PATH /usr/lib/arm-linux-gnueabihf/pkgconfig/
+
+
+# Disallow the `pkg-config` crate to look for the config for zlib, because build.rs of `libz-sys`
+# gets confused and pulls the system-wide library (i.e. of the host) instead of the target when
+# cross-compiling. This essentially leads to static linking of zlib.
+#
+# Alternatively, we can supply LIBZ_SYS_STATIC=1. Weirdly enough, installing libgtk-3-dev:armhf
+# also solves the problem somehow.
+#
+# Here is the related issue: https://github.com/rust-lang/libz-sys/issues/49
+ENV ZLIB_NO_PKG_CONFIG 1
+
+# This is for compiling GUI apps.
+# RUN aptitude install -y libasound2-dev:armhf libgtk-3-dev:armhf libsdl2-dev:armhf
+
+RUN useradd rust --user-group --create-home --shell /bin/bash --groups sudo
+WORKDIR /home/rust/src