This commit adds a Makefile rule for installing the vendor directory.
Both the `build` and `docker-build` phony rules depend on the `vendor`
rule, which in turn depends on the `glide.lock` rule. This means that
any time the `glide.lock` file is updated, the vendor directory will be
re-installed.
The `build` Makefile target attempted to place the generated artifact in
`/build`, where it should have used a local directory instead. This
commit makes the directory configurable using the `OUT_DIR` argument,
and defaults it to ./_output
