Improve build image resolution from Dockerfile base images

monitoring/docker-exporter/exporter.py

@@ -167,36 +167,53 @@ def parse_dockerfile_for_image(dockerfile_path):
     if not os.path.exists(dockerfile_path):
         return None
 
-    image_name = None
-
     try:
+        arg_defaults = {}
+        last_from = None
         with open(dockerfile_path) as df:
             for line in df:
                 line = line.strip()
-                # Prefer LABEL with image if present
+                if not line or line.startswith("#"):
+                    continue
+
+                if line.upper().startswith("ARG "):
+                    arg_body = line[4:].strip()
+                    if "=" in arg_body:
+                        key, value = arg_body.split("=", 1)
+                        arg_defaults[key.strip()] = value.strip()
+                    continue
+
+                # Prefer LABEL with image if present.
                 if "LABEL" in line and "image=" in line:
                     match = re.search(r'image=["\']?([^"\']+)["\']?', line)
                     if match:
-                        image_name = match.group(1)
+                        image_name = normalize_image_name(substitute_dockerfile_args(match.group(1), arg_defaults))
                         logger.debug(f"Found LABEL image={image_name} in {dockerfile_path}")
                         return image_name
 
-            # If no LABEL, use the last FROM line as fallback
-            df.seek(0)
-            last_from = None
-            for line in df:
-                line = line.strip()
                 if line.upper().startswith("FROM "):
-                    parts = line.split()
+                    from_clause = line[5:].strip()
-                    if len(parts) >= 2:
+                    if from_clause.startswith("--"):
-                        last_from = parts[1]
+                        split_clause = from_clause.split(None, 1)
+                        if len(split_clause) < 2:
+                            continue
+                        from_clause = split_clause[1]
+
+                    parts = from_clause.split()
+                    if not parts:
+                        continue
+
+                    candidate = substitute_dockerfile_args(parts[0], arg_defaults)
+                    if candidate and candidate.lower() != "scratch":
+                        last_from = normalize_image_name(candidate)
+
         if last_from:
             logger.debug(f"Found base FROM {last_from} in {dockerfile_path}")
             return last_from
     except Exception as e:
         logger.debug(f"Error reading Dockerfile {dockerfile_path}: {e}")
 
-    return image_name
+    return None
 
 def normalize_image_name(image_name):
     if not image_name:
@@ -207,6 +224,38 @@ def normalize_image_name(image_name):
         return image_name
     return f"{image_name}:latest"
 
+def is_compose_build_placeholder(image_name, project_name):
+    if not image_name:
+        return False
+    candidate = str(image_name)
+    project_prefix = f"{project_name}-"
+    if candidate.startswith(project_prefix):
+        return True
+    # Keep backward-compatible behavior for historical default project prefix.
+    return candidate.startswith("core-")
+
+def substitute_dockerfile_args(value, arg_defaults):
+    if not value:
+        return value
+
+    pattern = re.compile(r"\$\{([^}]+)\}|\$([A-Za-z_][A-Za-z0-9_]*)")
+
+    def replacer(match):
+        expr = match.group(1)
+        simple = match.group(2)
+        if simple:
+            return arg_defaults.get(simple, "")
+
+        if ":-" in expr:
+            var_name, default_value = expr.split(":-", 1)
+            return arg_defaults.get(var_name, default_value)
+        if "-" in expr:
+            var_name, default_value = expr.split("-", 1)
+            return arg_defaults.get(var_name, default_value)
+        return arg_defaults.get(expr, "")
+
+    return pattern.sub(replacer, value)
+
 def expand_compose_path(path_value, project_root):
     raw = str(path_value)
     raw = raw.replace("${PROJECT_ROOT}", project_root).replace("$PROJECT_ROOT", project_root)
@@ -302,6 +351,10 @@ def parse_compose_services(compose_files, project_name, project_root):
                     from_dockerfile = normalize_image_name(parse_dockerfile_for_image(dockerfile_path))
                     local_built_image = resolve_local_build_image(svc_name, project_name)
 
+                placeholder_image = is_compose_build_placeholder(image, project_name) or is_compose_build_placeholder(local_built_image, project_name)
+                if placeholder_image:
+                    resolved_image = from_dockerfile or image or local_built_image or f"{project_name}-{svc_name}:latest"
+                else:
                     resolved_image = image or local_built_image or from_dockerfile or f"{project_name}-{svc_name}:latest"
 
                 svc_map[svc_name] = {