new: dev: provide ``get_host_path`` to find host path of a local directory.

This is useful when preparing to launch a docker container from
a ``compose`` in docker command.

bin/compose-core

@@ -2444,6 +2444,42 @@ get_master_services() {
 export -f get_master_services
 
 
+get_current_docker_container_id() {
+    local line
+    line=$(cat "/proc/self/cpuset") || return 1
+    [[ "$line" == *docker* ]] || return 1
+    echo "${line##*/}"
+}
+export -f get_current_docker_container_id
+
+
+## if we are in a docker compose, we might want to know what is the
+## real host path of some local paths.
+get_host_path() {
+    local path="$1"
+    path=$(realpath "$path") || return 1
+    container_id=$(get_current_docker_container_id) || {
+        print "%s" "$path"
+        return 0
+    }
+    biggest_dst=
+    current_src=
+    while read-0 src dst; do
+        [[ "$path" == "$dst"* ]] || continue
+        if [[ "${#biggest_dst}" < "${#dst}" ]]; then
+            biggest_dst="$dst"
+            current_src="$src"
+        fi
+    done < <(get_volumes_for_container  "$container_id")
+    if [ "$current_src" ]; then
+        printf "%s" "$current_src"
+    else
+        return 1
+    fi
+}
+export -f get_host_path
+
+
 _setup_state_dir() {
     export state_tmpdir=$(mktemp -d -t tmp.XXXXXXXXXX)
     #debug "Creating temporary state directory in '$state_tmpdir'."