utils/docker-run: make it work in workdirs/woktrees

It is quite customary to use a single repository with multiple workdirs,
one for each active branch, with either the aging 'git new-workdir' or
the more recent 'git worktree'.

However, in a workdir/worktree, most entries in .git/ are only symlinks
to the actual files in the main repository.

Currently, utils/docker-run only bind-mounts the current working copy.
If that is a workdir/worktree, then it is going to be missing the actual
git data, resulting in errors like:

    $ ./utils/docker-run make check-package
    fatal: not a git repository (or any parent up to mount point [....]/buildroot)
    Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
    No files to check style
    make: *** [Makefile:1257: check-package] Error 1

So, we also bind-mount the actual git directory. If that is a subdir
of the current working copy, then it is already mounted and thus the
bind-mount is superfluous but harmless; for simplicity, we mount it
unconditionally.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Ricardo Martincoski <ricardo.martincoski@datacom.com.br>

utils/docker-run

@@ -2,6 +2,8 @@
 set -o errexit -o pipefail
 DIR=$(dirname "${0}")
 MAIN_DIR=$(readlink -f "${DIR}/..")
+# GIT_DIR to support workdirs/worktrees
+GIT_DIR="$(dirname "$(realpath "${MAIN_DIR}/.git/config")")"
 # shellcheck disable=SC2016
 IMAGE=$(grep ^image: "${MAIN_DIR}/.gitlab-ci.yml" | \
         sed -e 's,^image: ,,g' | sed -e 's,\$CI_REGISTRY,registry.gitlab.com,g')
@@ -11,6 +13,7 @@ declare -a docker_opts=(
     --rm
     --user "$(id -u):$(id -g)"
     --mount "type=bind,src=${MAIN_DIR},dst=${MAIN_DIR}"
+    --mount "type=bind,src=${GIT_DIR},dst=${GIT_DIR}"
     --workdir "${MAIN_DIR}"
 )
 if tty -s; then