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/centos/Dockerfile

@@ -1,29 +1,32 @@
 FROM centos:latest
 WORKDIR /build
+
 # install tools and dependencies
 RUN yum -y update&& \
     yum install -y git make gcc-c++ gcc file binutils
+
 # install rustup
 RUN curl -sSf https://static.rust-lang.org/rustup.sh -o rustup.sh &&\
 ls&&\
  sh rustup.sh --disable-sudo
+
 # show backtraces
 ENV RUST_BACKTRACE 1
+
 # set compiler
 ENV CXX g++
 ENV CC gcc
+
 # show tools
 RUN rustc -vV && \
 cargo -V && \
 gcc -v &&\
 g++ -v
+
 # build parity
-RUN git clone https://github.com/paritytech/parity && \
-	cd parity&&\
-	git checkout beta && \
-        git pull && \
-        ls -a&&\
-        cargo build --release --verbose && \
+ADD . /build/parity
+RUN cd parity&&\
+    cargo build --release --verbose && \
 	ls /build/parity/target/release/parity &&	\
 	strip /build/parity/target/release/parity