Add Dockerfile for ARM64 build, builds OK, but is slooooow, not tested

contrib/docker/Dockerfile.ubuntuARMviaEmulation

@@ -0,0 +1,55 @@
+
+# Beware, uses emulation, with AMD 24 threads: Building 6047.7s
+# Use: docker build . --platform=linux/arm64 --pull -t darkfi:ubuntu_arm64 -f contrib/docker/Dockerfile.ubuntuARMviaEmulation
+#   optionally with: --build-arg BUILD_OS_VER=20.04 --build-arg RUST_VER=1.65
+
+ARG RUST_VER=1.67       # stable nightly beta
+ARG BUILD_OS_VER=22.04  # 20.04
+ARG RUN_OS_VER=${BUILD_OS_VER}
+ARG REPOSITORY=ubuntu
+
+FROM --platform=$TARGETPLATFORM ${REPOSITORY}:${BUILD_OS_VER} as rust_builder
+ARG TARGETPLATFORM
+ARG BUILDPLATFORM
+ARG RUST_VER
+RUN echo "I am running on $BUILDPLATFORM, building for $TARGETPLATFORM"
+
+RUN apt-get update && DEBIAN_FRONTEND=noninteractive TZ=Etc/UTC apt-get install -y \
+  build-essential cmake jq wget curl \
+  pkg-config clang libclang-dev llvm-dev libudev-dev libfreetype6-dev \
+  libexpat1-dev curl gcc make libssl-dev fonts-lato libfontconfig-dev
+
+RUN curl https://sh.rustup.rs -sSf | bash -s -- -y --default-toolchain "${RUST_VER}"
+
+ENV PATH="/root/.cargo/bin:${PATH}"
+
+RUN rustup target add wasm32-unknown-unknown
+RUN rustup target add aarch64-unknown-linux-gnu
+RUN cargo search whatever  # creates .cargo cache
+RUN cargo install -f cross
+
+# 2. stage
+FROM --platform=$TARGETPLATFORM  rust_builder as builder
+
+WORKDIR /opt/darkfi
+
+COPY . /opt/darkfi
+
+RUN make clean
+
+RUN rm -rf ./target/*
+
+RUN bash -c 'make -j test &&  make -j all'
+
+# 3. stage
+FROM --platform=$TARGETPLATFORM ${REPOSITORY}:${RUN_OS_VER}
+
+RUN apt-get -y update && apt-get install -y openssl fonts-lato \
+  && apt-get clean && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /opt/darkfi
+
+COPY --from=builder /opt/darkfi/drk /opt/darkfi/darkfid  \
+   /opt/darkfi/ircd /opt/darkfi/dnetview /opt/darkfi/faucetd \
+   /opt/darkfi/zkas /opt/darkfi/vanityaddr  ./
+