build: Fix build-docker to include the full context. (#7114) (#7116)
Fixes#7068. The build-docker rule relies on being able to run make
build-linux, but did not pull the Makefile into the build context.
There are various ways to fix this, but this was probably the smallest.
(cherry picked from commit 6538776e6a)
Co-authored-by: M. J. Fromberger <fromberger@interchain.io>