new: ``!expand-vars`` tag for relations and options.

Will interpret any code like if it was in a shell here-doc string.
This will interpret basically ``$VAR`` and ``$()`` constructs.

bin/compose-core

@@ -1513,7 +1513,7 @@ export -f setup_host_resources
 
 relation-get () {
     local key="$1" out
-    if ! out=$(cat "$RELATION_DATA_FILE" | shyaml get-value "$key" 2>/dev/null); then
+    if ! out=$(cat "$RELATION_DATA_FILE" | shyaml -y get-value "$key" 2>/dev/null); then
         err "The key $WHITE$key$NORMAL was not found in relation's data."
         return 1
     fi
@@ -1522,6 +1522,35 @@ relation-get () {
 export -f relation-get
 
 
+expand_vars() {
+    local unlikely_prefix="UNLIKELY_PREFIX"
+    content=$(cat -)
+    ## find first identifier not in content
+    remaining_lines=$(echo "$content" | grep "^$unlikely_prefix")
+    while [ "$(echo "$remaining_lines" | grep "^$unlikely_prefix$")" ]; do
+        size_prefix="${#unlikely_prefix}"
+        first_matching=$(echo "$remaining_lines" |
+                             grep -v "^$unlikely_prefix$" |
+                             uniq -w "$((size_prefix + 1))" -c |
+                             sort -rn |
+                             head -n 1)
+        first_matching=${first_matching#"${x%%[![:space:]]*}"}
+        first_matching="${first_matching#* }"
+        next_char=${first_matching:$size_prefix:1}
+        if [ "$next_char" != "0" ]; then
+            unlikely_prefix+="0"
+        else
+            unlikely_prefix+="1"
+        fi
+        remaining_lines=$(echo "$remaining_lines" | grep "^$unlikely_prefix")
+    done
+    eval "cat <<$unlikely_prefix
+$content
+$unlikely_prefix"
+}
+export -f expand_vars
+
+
 yaml_get_interpret() {
     local content tag
     content=$(cat -)
@@ -1539,6 +1568,12 @@ yaml_get_interpret() {
                 return 1
             }
             ;;
+        "!var-expand")
+            echo "$content" | expand_vars || {
+                err "shell expansion failed"
+                return 1
+            }
+            ;;
         *)
             err "Invalid object tag ${WHITE}$tag${NORMAL}"
             return 1
@@ -1573,7 +1608,7 @@ export -f relation-base-compose-get
 
 relation-target-compose-get () {
     local key="$1" out
-    if ! out=$(echo "$RELATION_TARGET_COMPOSE_DEF" | shyaml get-value "options.$key" 2>/dev/null); then
+    if ! out=$(echo "$RELATION_TARGET_COMPOSE_DEF" | shyaml -y get-value "options.$key" 2>/dev/null); then
         err "The key $WHITE$key$NORMAL was not found in base service compose definition.."
         return 1
     fi