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

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