docker: attempt to return to dockerignore whitelist

previously the COPY instruction did not copy any files
when autobuilding on hub.docker.com

local builds worked as expected

.dockerignore

@@ -1,4 +1,5 @@
-/.git/
-/.gitignore
-/Dockerfile
-/README.md
+*
+!Makefile
+!src/
+
+# https://docs.docker.com/engine/reference/builder/#dockerignore-file