Update golang to 1.11.4, add gotestsum, bump golangci_lint version (#19)

Author: rfay

Dockerfile

@@ -1,10 +1,13 @@
-FROM golang:1.11.2-alpine
-ENV GOLANGCI_LINT_VERSION v1.12.3
+FROM golang:1.11.4-alpine
+ENV GOLANGCI_LINT_VERSION v1.12.5
+ENV GOTESTSUM_VERSION 0.3.2
 
 RUN apk update && apk add git bash build-base curl
 
 RUN curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | bash -s -- -b $GOPATH/bin $GOLANGCI_LINT_VERSION
 
+RUN curl -sSL "https://github.com/gotestyourself/gotestsum/releases/download/v$GOTESTSUM_VERSION/gotestsum_${GOTESTSUM_VERSION}_linux_amd64.tar.gz" | tar -xz -C /usr/local/bin gotestsum && chmod +x /usr/local/bin/gotestsum
+
 # discussion of various tools at
 # http://blog.ralch.com/tutorial/golang-tools-inspection/
 # https://dominik.honnef.co/posts/2014/12/go-tools/#goreturns