chore: Add /.git/ to the container build context (#203)

Without it the `version_with_commit_id` macro cannot infer what commit
the developer tools are installed from. The purpose of having this
information is "to make it possible for users to communicate what
version of the programs they are using when they experience issues"
(#137).

Drawback of adding the git directory to the container build context
include:
- Worse cache performance (if memory serves any file changes to the
  mounted tree will invalidate that step in the dockerfile). But since
  this is the last step and I don't usually rebuild the dev container
  unless I expect something to be different, I don't think this will be
  an issue.
- The resulting environment could in theory be the function of
  information not in the current commit or one of its ancestors. The
  likelihood and impact of this are both low and I think the benefit
  of the improved troubleshooting far outweighs the risk.

Author: apljungquist

.dockerignore

@@ -1,5 +1,6 @@
 *
 !/.devhost/
+!/.git/
 !/Cargo.lock
 !/Cargo.toml
 !/about.hbs