Reuse same dockerfile gen logic between prelude and steps

As now we build only when necessary, we need to make sure the images are
built similarly. The discrepancies between the two are less now, and
they can share the same logic.

This fixes a regresion where when no prelude is defined, the build
context isn't copied over

pkg/compiler/backend/simpledocker_test.go

@@ -87,6 +87,7 @@ ENV PACKAGE_CATEGORY=app-admin`))
 			Expect(err).ToNot(HaveOccurred())
 			Expect(dockerfile).To(Equal(`
 FROM luet/base
+COPY . /luetbuild
 WORKDIR /luetbuild
 ENV PACKAGE_NAME=enman
 ENV PACKAGE_VERSION=1.4.0