Зачем это все?

find /gitlab-runner/builds -type f -ctime +7 -delete && find /gitlab-runner/builds -type d -empty -delete

This work for me:

find /home/gitlab-runner/builds -type d -ctime +7 -not -path 'master' -not -path 'develop' -exec rm -Rf {} +

I prefer to keep the master and develop folders.

I suggest to add some depth to this solution.

find /home/gitlab-runner/builds -depth 2 -type d -ctime +7 -not -path 'master' -not -path 'develop' -exec rm -Rf {} +

The original script makes more sense than the ones in comments. There should be mindepth/maxdepth set, or otherwise it will clean partly inside the build directory, for example .git

#!/bin/bash
# cleanup_gitlab_builds: to remove old gitlab builds

WORK_DIR=/builds
LAST_MODIFY_DAYS=30 # no of days the directory was modified
dir_array=()
space_cleared=0

SUMMARY=1
VERBOSE=0
DRY_RUN=0

function num_fmt_cmd() {
    if command -v numfmt >/dev/null; then
        numfmt --to=iec-i --suffix=B --from-unit=1Ki --format='%9.2f' "$1"
    else
        # numfmt compatible output
        echo "${1}KiB"
    fi
}

set -e # exit on errors

# Handle cmd line options
while getopts "w:d:nqvsh" opt; do
    case "$opt" in
        w) WORK_DIR="$OPTARG" ;;
        d) LAST_MODIFY_DAYS="$OPTARG" ;;
        n) DRY_RUN=1 ;;
        v)
            VERBOSE=1
            SUMMARY=1
            ;;
        q)
            VERBOSE=0
            SUMMARY=0
            ;;
        *) ;;
    esac
done

if [ ! -d "$WORK_DIR" ]; then
    echo "$WORK_DIR does not exist - exiting!" >&2
    exit 1
fi

# https://stackoverflow.com/a/23357277/4005566
while IFS= read -r -d $'\0'; do
    if [[ $(find "$REPLY" -maxdepth 0 -ctime +"${LAST_MODIFY_DAYS}" -print) ]]; then
        dir_array+=("$REPLY")
    fi
done < <(find "$WORK_DIR" -type d -exec test -d {}/.git \; -prune -print0)

for dir in "${dir_array[@]}"; do
    dir_size=$(du -d 0 "$dir" | cut -f 1)
    [ "$VERBOSE" -eq 1 ] && echo "$dir  $(num_fmt_cmd "$dir_size")"
    if [ "$DRY_RUN" -eq 0 ]; then
        rm -r "$dir"
    fi
    space_cleared=$((space_cleared + dir_size))
done

if [ "$SUMMARY" -eq 1 ]; then
    [ "$DRY_RUN" -eq 1 ] && echo -n "dry-run: "
    echo "$(num_fmt_cmd "$space_cleared") cleared"
fi

I updated the script.

1. added a parameter for WORK_DIR (we use /builds to match inside and outside of gitlab-runner docker container)
2. fixed the find part - min-/maxdepth doesn't work with groups in groups
   1. print directories containing a .git directory
   2. ctime check doesn't work combined in one find -> second find required as ctime check
3. added a dry-run estimate summary output
4. added a check that WORK_DIR exists
5. changed the dry-run/quiet conditions to 1 as true and 0 to false and removed the "not"
6. UPDATE: added numfmt to format output with echo as fallback
7. UPDATE: added verbose/summary option for better output control
8. UPDATE: removed "echo" from rm - missed to remove after debugging