Add gotty to Dockerfile.

Dockerfile

@@ -1,13 +1,20 @@
 FROM golang:alpine as builder
 RUN apk update && \
     apk add --virtual build-deps make git
+# Build gotty
+RUN go get -d github.com/yudai/gotty && \
+    git -C /go/src/github.com/yudai/gotty checkout release-1.0 && \
+    go get github.com/yudai/gotty
+# Build Elvish
 COPY . /go/src/github.com/elves/elvish
 RUN make -C /go/src/github.com/elves/elvish get
 
 FROM alpine
 COPY --from=builder /go/bin/elvish /bin/elvish
+COPY --from=builder /go/bin/gotty /bin/gotty
 RUN adduser -D elf
 RUN apk update && apk add tmux man man-pages vim curl
 USER elf
 WORKDIR /home/elf
+EXPOSE 8080
 CMD ["/bin/elvish"]