build: Move the "full" Dockerfile into the root of our main repo

Previously this lived in a separate repo docker-hub-images, shared with
other products. Having multiple products sharing a repo complicates
release processes and works against some assumptions made by the Dockerhub
build system.

The "full" Dockerfile contains a Terraform binary built from source, and
so it can be used with any arbitrary Terraform source tree. This doesn't
yet include the "light" Dockerfile, which instead makes an image from
official binaries already on releases.hashicorp.com. That will follow in
a subsequent commit.

Dockerfile

@@ -0,0 +1,23 @@
+# This Dockerfile builds on golang:alpine by building Terraform from source
+# using the current working directory.
+#
+# This produces a docker image that contains a working Terraform binary along
+# with all of its source code, which is what gets released on hub.docker.com
+# as terraform:full. The main releases (terraform:latest, terraform:light and
+# the release tags) are lighter images including only the officially-released
+# binary from releases.hashicorp.com; these are built instead from
+# scripts/docker-release/Dockerfile-release.
+
+FROM golang:alpine
+MAINTAINER "HashiCorp Terraform Team <terraform@hashicorp.com>"
+
+RUN apk add --update git bash openssh
+
+ENV TF_DEV=true
+
+WORKDIR $GOPATH/src/github.com/hashicorp/terraform
+COPY . .
+RUN /bin/bash scripts/build.sh
+
+WORKDIR $GOPATH
+ENTRYPOINT ["terraform"]