Add docker build script to inject version information

Signed-off-by: Steve Coffman <steve@khanacademy.org>

Dockerfile

@@ -48,6 +48,9 @@ RUN go mod download
 
 COPY . .
 
+ARG goflags_arg # default intentionally unset
+ENV GOFLAGS=$goflags_arg
+
 RUN go install -v ./cmd/...
 
 FROM alpine:3.11

build_docker.sh

@@ -0,0 +1,31 @@
+#!/usr/bin/env sh
+
+#
+# Runs `go build` with flags configured for docker distribution. All
+# it does differently from `go build` is burn git commit and version
+# information into the binaries inside docker, so that we can track down user
+# issues.
+#
+############################################################################
+#
+# WARNING: Tailscale is not yet officially supported in Docker,
+# Kubernetes, etc.
+#
+# It might work, but we don't regularly test it, and it's not as polished as
+# our currently supported platforms. This is provided for people who know
+# how Tailscale works and what they're doing.
+#
+# Our tracking bug for officially support container use cases is:
+#    https://github.com/tailscale/tailscale/issues/504
+#
+# Also, see the various bugs tagged "containers":
+#    https://github.com/tailscale/tailscale/labels/containers
+#
+############################################################################
+
+set -eu
+
+eval $(./version/version.sh)
+
+GOFLAGS='-tags xversion -ldflags '"-X tailscale.com/version.Long=${VERSION_LONG} -X tailscale.com/version.Short=${VERSION_SHORT} -X tailscale.com/version.GitCommit=${VERSION_GIT_HASH}"
+docker build --build-arg goflags_arg="'""${GOFLAGS}""'" -t tailscale:tailscale .