Dockerfile best practices: clarify behavior of pipefail in RUN steps

[rvsasseen]

File: develop/develop-images/dockerfile_best-practices.md

Please consider clarifying whether the pipefail option set by "RUN set -o pipefail &&" carries over to subsequent RUN commands. In a bash script the pipefail setting would carry over to subsequent commands, but it's not clear to me whether it would in a Dockerfile. I would guess maybe the answer is no because no files are affected by the setting and so there's nothing in the resulting layer that preserves the setting.

By the same token, would it work just to "RUN set -o pipefail" once and have it carry over to subsequent RUN commands, or is that not possible.

[thaJeztah]

Thanks for reporting; the set -o won't be inherited by following RUN steps. Each RUN runs in a new container, and because of that, changes that are only present in memory (like set, but also (e.g.) export SOMEVAR=somevalue) are not persisted. Changes that are made to the filesystem on the other hand are persisted, so would be available in RUN steps after that.

I agree that this may need to be explained better (contributions welcome); perhaps also in the Dockerfile reference; https://docs.docker.com/engine/reference/builder/#run (that file is maintained in the CLI repository though, so would need a separate pull request; https://github.com/docker/cli/blob/master/docs/reference/builder.md)

[docker-robot]

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[thaJeztah]

Looks like this closed before being improved; let me mark it as "frozen"