Add Dockerfile.self-contained (#283)

* Add Dockerfile using multi-stage build

* Rename Dockerfile and use more RUN commands

Dockerfile.self-contained

@@ -0,0 +1,38 @@
+#
+# Builder
+#
+
+FROM golang:alpine as builder
+
+# use version (for example "v0.3.3") or "master"
+ARG WATCHTOWER_VERSION=master
+
+RUN apk add --no-cache \
+    alpine-sdk \
+    ca-certificates \
+    git \
+    tzdata
+
+RUN git clone --branch "${WATCHTOWER_VERSION}" https://github.com/containrrr/watchtower.git
+
+RUN \
+  cd watchtower && \
+  \
+  GO111MODULE=on CGO_ENABLED=0 GOOS=linux go build -a -ldflags '-extldflags "-static"' . && \
+  GO111MODULE=on go test ./... -v
+
+
+#
+# watchtower
+#
+
+FROM scratch
+
+LABEL "com.centurylinklabs.watchtower"="true"
+
+# copy files from other container
+COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt
+COPY --from=builder /usr/share/zoneinfo /usr/share/zoneinfo
+COPY --from=builder /go/watchtower/watchtower /watchtower
+
+ENTRYPOINT ["/watchtower"]