add coursier to gitserver, repo-updater & combined Dockerfiles (RFC 307)
Created by: Strum355
As part of RFC 307, we're adding support for JVM artifact hosts as external service types to gitserver. Related PR can be found here https://github.com/sourcegraph/sourcegraph/pull/21703.
This PR adds bundling of the Coursier binary in the gitserver and all-in-one image, pinned to the 2.0.16 release. Coursier is used by us to communicate with Maven artifact hosts (such as Maven Central), fetch jar sources etc. All usages can be found here https://github.com/sourcegraph/sourcegraph/pull/21703/files#diff-f6650fc82f7e1706d81023230b9d12bf292463057ad9c3c2af23488482f0603c.
Closes #21881 (closed)
cc @sourcegraph/security if theres anything you need to do/check with regards to adding this
cc @sourcegraph/distribution if theres any other changes that need to be made to cover all deployment cases
cc @sourcegraph/code-intel for non-review