Add Alpine based Dockerfile

To build it:

```
docker build -f Dockerfile.alpine -t <image:tag> .
```

Signed-off-by: Waldemar Quevedo <wally@synadia.com>

Dockerfile.alpine

@@ -0,0 +1,22 @@
+FROM golang:1.11-alpine3.8 AS builder
+
+WORKDIR $GOPATH/src/github.com/nats-io/gnatsd
+
+MAINTAINER Waldemar Quevedo <wally@synadia.com>
+
+RUN apk add --update git
+
+COPY . .
+
+RUN CGO_ENABLED=0 go build -v -a -tags netgo -installsuffix netgo -ldflags "-s -w -X github.com/nats-io/gnatsd/server.gitCommit=`git rev-parse --short HEAD`" -o /gnatsd
+
+FROM alpine:3.8
+
+RUN apk add --update ca-certificates
+
+COPY --from=builder /gnatsd /gnatsd
+
+# Expose client, management, cluster and gateway ports
+EXPOSE 4222 8222 6222 5222
+
+ENTRYPOINT ["/gnatsd"]