Docker changes (#29)

* Update README.md

I needed to change 
-v .env:.env 
to
-v .env:/.env
to get it to work.
Docker (Linux) was not happy otherwise.

* Update Dockerfile

Added cmake to Dockerfile, so Pi4 (ARM chip) will work.

* Update README.md

Corrected volume mount

Dockerfile

@@ -10,6 +10,7 @@ RUN apt-get update \
 RUN DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
         python3 \
         build-essential \
+        cmake \
     && apt clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
 
 WORKDIR /build