Bump Go to 1.9, switch base image to Alpine

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.4.2
+FROM golang:1.9-alpine3.6
 
 # Copy the local package files to the container's workspace.
 ADD . /go/src/github.com/martin61/i2p-tools
@@ -7,7 +7,9 @@ ADD . /go/src/github.com/martin61/i2p-tools
 WORKDIR /go/src/github.com/martin61/i2p-tools
 
 # Build everything
-RUN go get
+RUN apk add --no-cache --update git && \
+    go get && \
+    apk del git
 RUN go build -o /i2p-tools
 
 CMD ["/i2p-tools"]