Add repository_file attribute to the container_pull rule

[nlewo]

This attribute allows to set the repository value from a file as it is
already possible for the tag thanks to the tag_file attribute.

[nlewo]

Note we always need to provide the repository attribute because it is mandatory. Since for most use cases the repository attribute should be sufficient, i think it would be better to preserve this behavior.
WDYT?

[smukherj1]

Seems reasonable to have repository_file given tag_file already exists. Feel free to update the pusher binary to implement this.

[nlewo]

@smukherj1 What exactly should i implement in the pusher binary?
The repository_file (and tag_file) value is substituted by bash (in the generated bash script): the pusher takes as argument a string such as --dst=repository:$(cat ${RUNFILES}/tag), where $(cat ${RUNFILES}/tag) is evaluated by bash.

[smukherj1]

Apologies, I missed the cat.

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[smukherj1]

/gcbrun

[nlewo]

Thanks ;)