Closes#69930. Closes#69928.
The ES build currently has 2 types of Docker output - Docker images,
and Docker build contexts. At the moment, only the images are tested,
meaning that bugs in the build contexts can go unnoticed.
This PR changes how we create Docker images so that we first create
the build contexts, and then build the images using them. This does
require some sleight-of-hand - the build contexts expect to download
an Elasticsearch archive directorly from the `Dockerfile`, which
will only ever work for non-snapshot version builds. In order to
get around this, the `Dockerfile` is modified to `COPY` in a local
archive file. Any other dependency files must exist in the build
context archive.
This PR also builds and tests the Iron Bank context. We do not
currently build a Docker image for this at all, and to build an
image requires us to set some build arguments to useful values. We
also need to provide all artifacts to the build, as the `Dockerfile`
cannot download anything. As a result, the `:distribution:docker`
project now defines a GitHub repository so that Gradle will download
a `tini` binary.
Note that there will need to be corresponding changes to
`release-manager`.
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