Use more shell-agnostic syntax for docker login

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What does this MR do?

In docs: Pass Docker credentials in a safer way (!142178 - merged) we changed the syntax in a bunch of docker login examples to something that doesn't work in plain sh. However, a lot of common Docker images only have sh available. Combining the syntax we now document with other basic examples from our own documentation can easily result in confusingly broken pipelines, see !142178 (comment 1869288236) for more details.

This MR suggests a different syntax that should still reach the same goal of avoiding possible token/password leakage as the original MR.

Related issues

!142178 (merged)

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