cmd/shfmt: add Dockerfile for Docker Hub

Pinning versions of the builder and final base image, so that its
behavior doesn't change over time.

Updates #68.

cmd/shfmt/Dockerfile

@@ -0,0 +1,11 @@
+FROM golang:1.11.2-alpine3.8
+
+ENV VERSION 2.6.1
+
+RUN apk add --no-cache git \
+      && go get -d mvdan.cc/sh/cmd/shfmt \
+      && git -C $GOPATH/src/mvdan.cc/sh checkout -q v$VERSION \
+      && CGO_ENABLED=0 go install -ldflags '-w -s -extldflags "-static"' mvdan.cc/sh/cmd/shfmt
+
+FROM busybox:1.29.3-musl
+COPY --from=0 /go/bin/shfmt /bin/shfmt