chore: remove unused historic image files

check_dashes.py

@@ -0,0 +1,21 @@
+import os
+import re
+
+count = 0
+for root, dirs, files in os.walk("/Users/baohua/Github/books/docker_practice"):
+    if ".git" in root or "node_modules" in root:
+        continue
+    for file in files:
+        if file.endswith(".md"):
+            filepath = os.path.join(root, file)
+            with open(filepath, "r", encoding="utf-8") as f:
+                lines = f.readlines()
+            
+            for i, line in enumerate(lines):
+                # match optional spaces, then exactly one dash, then no space and no dash
+                m = re.match(r'^(\s*)-([^- \t\n].*)$', line)
+                if m:
+                    print(f"{filepath}:{i+1}:{line.rstrip()}")
+                    count += 1
+
+print(f"Total found: {count}")