Dockerfile 里面 RUN npm install 后，找不到 node_modules？

Dockerfile 是这样的（直接复制的 node 官方的 Dockerfile 示例）
链接： https://nodejs.org/en/docs/guides/nodejs-docker-webapp/

#FROM node:12
FROM my-node-ubuntu-image

# Create app directory
WORKDIR /apps/express-upload

# Install app dependencies
# A wildcard is used to ensure both package.json AND package-lock.json are copied
# where available (npm@5+)
COPY package*.json ./

RUN npm install
# If you are building your code for production
# RUN npm ci --only=production

RUN ls -al

# Bundle app source
COPY . .

EXPOSE 8080
CMD [ "node", "app.js" ]

现在问题是：
使用官方一样的 base image node:12 就可以在 RUN ls -al 那一步看到有 node_modules 文件夹，
但是用我自己创建的 base image my-node-ubuntu-image (是装有 node 的)，就找不到 node_modules 文件夹了？

docker build 语句：

docker build . -t node-web-app:0.05

执行结果：（可以看到 npm install 是执行了的，但是下面的 RUN ls -al 没有 node_modules ）

Sending build context to Docker daemon  121.1MB
Step 1/8 : FROM my-node-ubuntu-image
 ---> 4d66ffd553f3
Step 2/8 : WORKDIR /apps/express-upload
 ---> Using cache
 ---> 61e3227e0d01
Step 3/8 : COPY package*.json ./
 ---> 47e0df84b2e3
Step 4/8 : RUN npm install
 ---> Running in 55437825f54f
npm WARN nodejs@1.0.0 No description
npm WARN nodejs@1.0.0 No repository field.

added 224 packages from 267 contributors and audited 224 packages in 8.51s
found 3 vulnerabilities (1 low, 2 moderate)
  run `npm audit fix` to fix them, or `npm audit` for details
Removing intermediate container 55437825f54f
 ---> a8e5ce1df24a
Step 5/8 : RUN ls -al
 ---> Running in a96450a9b1ba
total 76
drwxr-xr-x 2 root root  4096 Dec  3 07:17 .
drwxr-xr-x 3 root root  4096 Dec  3 07:17 ..
-rw-r--r-- 1 root root 63608 Dec  1 18:34 package-lock.json
-rw-r--r-- 1 root root   615 Dec  3 06:24 package.json
Removing intermediate container a96450a9b1ba
 ---> 12e7d8dea420
Step 6/8 : COPY . .
 ---> 60aa2971f746
Step 7/8 : EXPOSE 8080
 ---> Running in d9693d5e39c4
Removing intermediate container d9693d5e39c4
 ---> 3039a0c5d66c
Step 8/8 : CMD [ "node", "app.js" ]
 ---> Running in e925d8d9e250
Removing intermediate container e925d8d9e250
 ---> 69941e3a6915
Successfully built 69941e3a6915
Successfully tagged node-web-app:0.05