こちらのページにあるのと同様のことを行いました。
Flask を Docker で動かします。
labs/beginner/chapters/webapps.md
2.2 Docker Images

次のファイルを作成

$ tree
.
├── app.py
├── Dockerfile
├── requirements.txt
└── templates
    └── index.html

app.py

from flask import Flask, render_template
import random

app = Flask(__name__)

# list of cat images
images = [
	"fmervo000001gsle.jpg",
	"rn2ola000000lk6e.jpg",
	"rn2ola000001gogf.jpg",
	"6fujishigai_s_s.jpg",
	"rn2ola000000lk6r.jpg",
	"5fujikawarakuza_s_s.jpg"
	]

url_base = "https://www.city.fuji.shizuoka.jp/page/gazou/fmervo000001dsro-img/"



@app.route('/')
def index():
	url = url_base + random.choice(images) 
	return render_template('index.html', url=url)

if __name__ == "__main__":
	app.run(host="0.0.0.0")

Dockerfile

# our base image
FROM alpine:3.13

# Install python and pip
RUN apk add --update py3-pip

# install Python modules needed by the Python app
COPY requirements.txt /usr/src/app/
RUN pip install --no-cache-dir -r /usr/src/app/requirements.txt

# copy files required for the app to run
COPY app.py /usr/src/app/
COPY templates/index.html /usr/src/app/templates/

# tell the port number the container should expose
EXPOSE 5000

# run the application
CMD ["python3", "/usr/src/app/app.py"]

requirements.txt

Flask==2.2.2

templates/index.html

<!DOCTYPE html>
<html lang="ja">
<head>
<meta http-equiv="CONTENT-TYPE" content="text/html; charset=utf-8" />
<title>富士山</title>
</head>
<body>
<div class="container">
<p>富士山 航空写真</p>
<blockquote>
<img src="{{url}}" />
</blockquote>
<p>Courtesy: <a href="https://www.city.fuji.shizuoka.jp/page/gazou/fmervo000001dsro.html">富士市</a></p>
</div>
Jul/2/2021 AM 11:29<br />
</body>
</html>

Flask として問題のないことを確認

$ python app.py 
 * Serving Flask app 'app' (lazy loading)
 * Environment: production
   WARNING: This is a development server. Do not use it in a production deployment.
   Use a production WSGI server instead.
 * Debug mode: off
 * Running on all addresses.
   WARNING: This is a development server. Do not use it in a production deployment.
 * Running on http://192.168.1.5:5000/ (Press CTRL+C to quit)

ブラウザーで
http://localhost:5000/
にアクセス

Docker image の作成

docker build -t uchida/myfirstapp .

$ docker images
REPOSITORY          TAG       IMAGE ID       CREATED          SIZE
uchida/myfirstapp   latest    71f862cd6bd4   34 seconds ago   75MB
alpine              3.13      6dbb9cc54074   2 months ago     5.61MB

起動

docker run -p 8888:5000 --name myfirstapp uchida/myfirstapp

再起動

docker run -p 8888:5000 uchida/myfirstapp

ブラウザーで
http://localhost:8888/
にアクセス

確認したバージョン

$ docker --version
Docker version 20.10.21, build 20.10.21-0ubuntu3