build: use docker for golang

As discussed in #1639.

Note that .dockerignore had to be changed as to *not* skip what's needed
for the build: vendor, obviously, and .git for the compiled-in version
information.

parts:
- .travis.yml: don't bother about golang anymore
- docs/devel/DEVELOPMENT.md: update
- Makefile: update hint

Signed-off-by: Stephan Renatus <srenatus@chef.io>

Dockerfile

@@ -0,0 +1,21 @@
+# Copyright 2019 The OPA Authors.  All rights reserved.
+# Use of this source code is governed by an Apache2
+# license that can be found in the LICENSE file.
+ARG BUILD_COMMIT
+# we cant use build-args in `COPY --from=...` below, so work around this
+# see: https://medium.com/@tonistiigi/advanced-multi-stage-build-patterns-6f741b852fae
+FROM build-${BUILD_COMMIT} AS copy-src
+
+FROM gcr.io/distroless/base${VARIANT}
+# make root (uid 0) default when not specified
+ARG USER=0
+MAINTAINER Torin Sandall <torinsandall@gmail.com>
+COPY --from=copy-src /go/src/github.com/open-policy-agent/opa/opa_linux_amd64 /opa
+
+# Any non-zero number will do, and unfortunately a named user will not,
+# as k8s pod securityContext runAsNonRoot can't resolve the user ID:
+# https://github.com/kubernetes/kubernetes/issues/40958
+USER ${USER}
+
+ENTRYPOINT ["/opa"]
+CMD ["run"]