Pass go build tags from environment.

Can now do something like `make VK_BUILD_TAGS=no_jaeger_exporter build`
and jaeger support will not be compiled in.

Currently there are build flags to omit every provider and every (the
only) supported tracing exporter.

Dockerfile

@@ -4,28 +4,23 @@ ENV PATH /go/bin:/usr/local/go/bin:$PATH
 ENV GOPATH /go
 
 RUN	apk add --no-cache \
-	ca-certificates
-
-COPY . /go/src/github.com/virtual-kubelet/virtual-kubelet
-
-RUN set -x \
-	&& apk add --no-cache --virtual .build-deps \
+	ca-certificates \
+	--virtual .build-deps \
 		git \
 		gcc \
 		libc-dev \
 		libgcc \
-        make \
-	&& cd /go/src/github.com/virtual-kubelet/virtual-kubelet \
-	&& make build \ 
-	&& apk del .build-deps \
-    && cp bin/virtual-kubelet /usr/bin/virtual-kubelet \
-	&& rm -rf /go \
-	&& echo "Build complete."
+        make
+
+COPY . /go/src/github.com/virtual-kubelet/virtual-kubelet
+WORKDIR /go/src/github.com/virtual-kubelet/virtual-kubelet
+ARG BUILD_TAGS="netgo osusergo"
+RUN make VK_BUILD_TAGS="${BUILD_TAGS}" build
+RUN cp bin/virtual-kubelet /usr/bin/virtual-kubelet
+
 
 FROM scratch
-
 COPY --from=builder /usr/bin/virtual-kubelet /usr/bin/virtual-kubelet
 COPY --from=builder /etc/ssl/certs/ /etc/ssl/certs
-
 ENTRYPOINT [ "/usr/bin/virtual-kubelet" ]
 CMD [ "--help" ]