Fix sed regex hyphen placement

`sed` complained about the former rule contained an illegal charanger
range. According to [0], 9.3.5 RE Bracked Expression (7) one's supposed
to have the - be either first or last for it to be treated literally.
This is also different from some languages, e.g. python, in which the
previous regex works just fine.

[0]: https://www2.cs.sfu.ca/~anoop/courses/CMPT-379-Fall-2004/posix-regexp.html

(cherry picked from commit 6b5c18c)

Author: Martin Hafskjold Thoresen

Diff for: docker/build-judgehost.sh

@@ -10,7 +10,7 @@ docker_tag="$1"
 docker build -t "${docker_tag}-build" -f judgehost/Dockerfile.build .
 
 # Build chroot
-builder_name=$(echo "${docker_tag}" | sed 's/[^a-zA-Z0-9\-_]/-/g')
+builder_name=$(echo "${docker_tag}" | sed 's/[^a-zA-Z0-9_-]/-/g')
 docker rm -f "${builder_name}" > /dev/null 2>&1 || true
 docker run --name "${builder_name}" --cap-add=sys_admin "${docker_tag}-build"
 docker cp "${builder_name}:/chroot.tar.gz" .