dockerfile: Add Dockerfile (#4733)

This adds a multi stage build Dockerfile. The end state is equivalent to
the current syncthing/docker repository (which will be retired).

Dockerfile

@@ -0,0 +1,29 @@
+FROM golang:1.9 AS builder
+
+WORKDIR /go/src/github.com/syncthing/syncthing
+COPY . .
+
+ENV CGO_ENABLED=0
+ENV BUILD_HOST=syncthing.net
+ENV BUILD_USER=docker
+RUN rm -f syncthing && go run build.go build syncthing
+
+FROM alpine
+
+RUN apk add --no-cache ca-certificates
+
+COPY --from=builder /go/src/github.com/syncthing/syncthing/syncthing /bin/syncthing
+
+RUN echo 'syncthing:x:1000:1000::/var/syncthing:/sbin/nologin' >> /etc/passwd \
+    && echo 'syncthing:!::0:::::' >> /etc/shadow \
+    && mkdir /var/syncthing \
+    && chown syncthing /var/syncthing
+
+USER syncthing
+ENV STNOUPGRADE=1
+
+HEALTHCHECK --interval=1m --timeout=10s \
+  CMD nc -z localhost 8384 || exit 1
+
+ENTRYPOINT ["/bin/syncthing", "-home", "/var/syncthing/config", "-gui-address", "0.0.0.0:8384"]
+