Remove duplicated stage from Dockerfile.alpine

[shouichi]

Building image fails with the following error.

$ docker build -t sops -f Dockerfile.alpine .
Sending build context to Docker daemon  884.2kB
Step 1/11 : FROM golang:1.12-alpine3.10 AS builder
 ---> 6d6865cf688e
builder stage name already used

[FnTm]

This should be a fix for #1022 .