add suitable .dockerignore

This reduces the `docker build` time spent in
`STEP 6: COPY . fineract` from about 6m to 3m,
because it avoids copying locally built artifacts
into the container (which we don't need there, we're
just about to re-build them in the container anyway).

This also provides better isolation from the local dev
environment and the should lead to more reproducible
container builds.

.dockerignore

@@ -0,0 +1,2 @@
+build/*
+.gradle/*