Re-add "Dockerfile" for base image in the root of our repository.

I've moved the "base" image Dockerfiles from the root to "/docker/base/" in vitessio#2769. This broke the automated build on Docker Hub because now it assumes "/docker/base" as build context and not "/" anymore. But "/" as build context is required to run "make build" and access the source code.

Therefore, I'm re-adding "Dockerfile" as symlink pointing to "docker/base/Dockerfile".

I'm also deleting "docker/base/README.md" again because I only added it for the automated build (it must be in the same directory as the "Dockerfile"). With the switch back to "/" as build context, it will use "/README.md" instead.

Dockerfile

@@ -0,0 +1 @@
+docker/base/Dockerfile

docker/base/Dockerfile

@@ -1,3 +1,11 @@
+# NOTE: This file is also symlinked as "Dockerfile" in the root of our
+#       repository because the automated build feature on Docker Hub does not
+#       allow to specify a different build context. It always assumes that the
+#       build context is the same directory as the Dockerfile is in.
+#       "make build" below must be called in our repository's root and
+#       therefore we need to have the symlinked "Dockerfile" in there as well.
+# TODO(mberlin): Remove the symlink and this note once
+# https://github.com/docker/hub-feedback/issues/292 is fixed.
 FROM vitess/bootstrap:mysql57
 
 # Re-copy sources from working tree