improved dockerfile builds (#5659)

* docker builds current branch instead of cloning the repo

* moved ADD command right before RUN cd parity in Dockerfiles

* update docker/README.md

* removed --no-cache from docker/README.md

docker/ubuntu/Dockerfile

@@ -1,5 +1,6 @@
 FROM ubuntu:14.04
 WORKDIR /build
+
 # install tools and dependencies
 RUN apt-get update && \
         apt-get install -y \
@@ -29,10 +30,8 @@ gcc -v &&\
 g++ -v
 
 # build parity
-RUN git clone https://github.com/paritytech/parity && \
-        cd parity && \
-        git checkout beta && \
-        git pull && \
+ADD . /build/parity
+RUN cd parity && \
         cargo build --release --verbose && \
         ls /build/parity/target/release/parity && \
         strip /build/parity/target/release/parity