add pkg-config and some configuration to Dockerfile.Rpi to allow cross-compilation

contrib/Dockerfile.Rpi

@@ -24,6 +24,7 @@ RUN apt-get update
 
 RUN apt-get install -y curl git build-essential crossbuild-essential-armhf
 RUN apt-get install -y libasound2-dev libasound2-dev:armhf
+RUN apt-get install -y pkg-config
 
 RUN curl https://sh.rustup.rs -sSf | sh -s -- -y
 ENV PATH="/root/.cargo/bin/:${PATH}"
@@ -35,9 +36,12 @@ RUN mkdir /.cargo && \
 RUN mkdir /build
 ENV CARGO_TARGET_DIR /build
 ENV CARGO_HOME /build/cache
+ENV PKG_CONFIG_PATH /usr/lib/arm-linux-gnueabihf/pkgconfig/
+ENV PKG_CONFIG_ALLOW_CROSS 1
 
 ADD . /src
 WORKDIR /src
+
 RUN cargo build --release --target arm-unknown-linux-gnueabihf --no-default-features --features "alsa-backend"