update Dockerfile

Author: wzshiming

Dockerfile

@@ -1,10 +1,10 @@
-FROM golang:1.10-alpine3.7 AS builder
+FROM golang:alpine AS builder
 WORKDIR /go/src/github.com/wzshiming/pic2ascii/
 COPY . .
 RUN apk add -U gcc libc-dev ffmpeg-dev ffmpeg-libs
-RUN go install -tags support_video -a -ldflags '-s -w'  ./cmd/...
+RUN go install -tags support_video -a -ldflags '-s -w' ./cmd/...
 
-FROM alpine:3.7
+FROM alpine
 COPY --from=builder /go/bin/pic2ascii /usr/local/bin/
 RUN apk add -U --no-cache ffmpeg-libs
 ENTRYPOINT [ "pic2ascii" ]