Use the --no-cache switch

Problematic code:

FROM alpine:3.7
RUN apk update \
    && apk add foo=1.0 \
    && rm -rf /var/cache/apk/*

FROM alpine:3.7
RUN apk add --update foo=1.0
    && rm -rf /var/cache/apk/*

Correct code:

FROM alpine:3.7
RUN apk --no-cache add foo=1.0

Rationale:

As of Alpine Linux 3.3 there exists a new --no-cache option for apk. It allows users to install packages with an index that is updated and used on-the-fly and not cached locally:

This avoids the need to use --update and remove /var/cache/apk/* when done installing packages.

inspired by https://github.com/gliderlabs/docker-alpine/blob/master/docs/usage.md#disabling-cache