When you build from a Dockerfile, it runs in a given "context"; that
is the directory the Dockerfile is in and the directories below it.
It can not access anything outside that context during the build.
When building a container for a project in the gate, you may wish to
install sibling projects that Zuul has checked-out into your container
(i.e. so that Depends-On works). As mentioned, because
/home/zuul/src/<project> is not in the context of the current project,
you will not be able to access this source code during the container
build.
So to help facilitate dependencies, add a siblings: tag which can copy
some or all of the required-projects already specified for the job
into a special sub-directory of the current source.
Because all the code is now in the same context, this will allow build
scripts to be written that look for directories in .zuul-siblings and
can install the source code from there. To further help the scripts,
the ZUUL_SIBLINGS arg is set for the docker build giving the copied
paths.
The test is updated with some paths to test the copy.
Change-Id: I079d823e7194e15b1b496aea0f53f70f6b563f02
0b0cb18a60