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Pin versions in apk add

Problematic code:

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

Correct code (Partial pin glob):

FROM alpine:3.7
RUN apk --no-cache add foo=~1.2.3

Correct code (Exact pin):

FROM alpine:3.7
RUN apk --no-cache add foo=1.2.3

Note: Pinning exact versions can cause future builds to suddenly fail if that version is no longer available. Use with caution.

Available versions in apk can search on https://pkgs.alpinelinux.org

Rationale:

https://docs.docker.com/develop/develop-images/instructions/#run

Version pinning forces the build to retrieve a limited range of versions, or an exact particular version, regardless of what’s in the cache. This technique can also reduce failures due to unanticipated changes in required packages.