Docker: fix list of executables

Context

This should fix an issue reported by @Ochem where Docker images for master do not contain experimental executables (like octez-baker-alpha).

The hypothesis is that this issue was introduced by !6962 (merged). The name of the variables that controls the list of executables changed during review but was not changed everywhere.

The fix was found by @arvidnl (thanks!).

Manually testing the MR

Check that the Docker images for this branch still work (see instructions in !6962 (merged)). For master, we'll probably just merge and see :p

Checklist

• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR