Docker: Copy irmin/ directory

Context

Following the MR vendoring Irmin, @MBourgoin noticed the jobs building Docker images was broken. The reason is simple: we don’t copy Irmin code in the build Docker image, thus leading to a situation where the Irmin vendored version is not present when trying to build within the docker image.

Manually testing the MR

Trust the CI jobs building Docker images.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR