Ensure remove any unused 'FROM' aliases (not used by 'COPY --from')

Unused FROM aliases can spot on aliases mistyping or a whole build stage that are not needed anymore.

Risk Level: medium
Platform: Docker
Spectral Rule ID: DOCKR013

REMEDIATION

remove unused FROM aliases.

- FROM golang:1.7.3 AS builder
+ FROM golang:1.7.3
WORKDIR /go/src/github.com/alexellis/href-counter/
RUN go get -d -v golang.org/x/net/html
COPY app.go    .
RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o app .

FROM alpine:latest
...

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