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

.dockerignore

@@ -0,0 +1,22 @@
+# Generated by Cargo
+# will have compiled files and executables
+target
+
+*.swp
+*.swo
+*.swn
+*.DS_Store
+
+# Visual Studio Code stuff
+.vscode
+
+# GitEye stuff
+.project
+
+# idea ide
+.idea
+
+# git stuff
+.git
+
+ethcore/res/ethereum/tests