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Do not use apk upgrade

Problematic code:

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

Correct code:

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

Rationale:

You should avoid RUN apk upgrade, as many of the “essential” packages from the parent images won’t upgrade inside an unprivileged container. If a package contained in the parent image is out-of-date, you should contact its maintainers. If you know there’s a particular package, foo, that needs to be updated, use apk --no-cache add foo to update automatically.

inspired by https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/#run