How to resolve issues with WARNING: Ignoring APKINDEX.40a3604f.tar.gz: No such file or directory?

Please, can someone help me? I am a beginner, i don’t know where i got it wrong. i’m trying to build my docker image but when i run: docker build ., i got the following errors:

ERROR: http://dl-cdn.alpinelinux.org/alpine/v3.12/main: temporary error (try again later)
WARNING: Ignoring APKINDEX.2c4ac24e.tar.gz: No such file or directory
ERROR: http://dl-cdn.alpinelinux.org/alpine/v3.12/community: temporary error (try again later)
WARNING: Ignoring APKINDEX.40a3604f.tar.gz: No such file or directory
2 errors; 35 distinct packages available
The command ‘/bin/sh -c apk update’ returned a non-zero code: 2

See below my Dockerfile file
FROM python:3.7-alpine
MAINTAINER Tboost Technology
#set environment variable
ENV PYTHONUNBUFFERED 1

            #install dependencies
            COPY ./requirements.txt /requirements.txt
            RUN apk update
            RUN rm -rf /var/cache/apk/* && \
                rm -rf /tmp/*
            RUN apk update \
                && apk add --update --no-cache postgresql-client
            RUN apk add --update --no-cache --virtual .tmp-build-deps \
                gcc libc-dev python3-dev linux-headers postgresql-dev
            RUN pip3 install -r /requirements.txt
            RUN apk del .tmp-build-deps

            #mkdir to store your apps source code
            RUN mkdir /app
            WORKDIR /app
            COPY ./app /app

            #create user to run apps in the docker
            RUN adduser -D user
            #switch to the user
            USER user

My docker-compose.yml file
version: “3”

services: 
    app:
        build: 
            context: .
        ports: 
            - "8000:8000"
        extra_hosts: 
            - "host.docker.internal:172.17.0.1"
        volumes: 
            - ./app:/app
        command: >
            sh -c "python manage.py runserver 0.0.0.0:8000"
        environment: 
            - DB_HOST=db
            - DB_NAME=app
            - DB_USER=postgres
            - DB_PASS=supersecretpassword
        depends_on: 
            - db
    db:
        image: postgres:11-alpine
        restart: always
        environment: 
            - POSTGRES_DB=app
            - POSTGRES_USER=postgres
            - POSTGRES_PASSWORD=supersecretpassword

and my requirements.txt file
Django>=3.0.7,<3.0.8
djangorestframework>=3.11.0,<3.12.0
psycopg2>=2.7.5,<2.8.0
flake8>=3.6.0,<3.7.0

i face the same problem ,have you resolve it

Your network or DNS resolution does not work I guess. Try netstat dl-cdn.alpinelinux.org to test DNS resolution and ping 8.8.8.8 to test if you have network connection.

You can use the netshoot image for that. It will not be as easy, but this is what I have time for now.