Docker: Add .dockerignore to setup clean workspace for docker build (#…

…122)

This improves local rebuilds cache if unrelated files are changed.

For example modifying entrypoint.sh does not invalidate rebuild of java
sources.

.dockerignore

@@ -0,0 +1,6 @@
+# https://docs.docker.com/engine/reference/builder/#dockerignore-file
+/*
+!/build.sbt
+!/docker/entrypoint.sh
+!/project
+!/src