Can't install pip packages inside a docker container with Ubuntu

I'm following the fig guide to using docker with a python application, but when docker gets up to the command

RUN pip install -r requirements.txt

I get the following error message:

Step 3 : RUN pip install -r requirements.txt
 ---> Running in fe0b84217ad1
Collecting blinker==1.3 (from -r requirements.txt (line 1))
  Retrying (Retry(total=4, connect=None, read=None, redirect=None)) after connection broken by 'ProtocolError('Connection aborted.', gaierror(-2, 'Name or service not known'))': /simple/blinker/

This repeats several times and then I get another message:

Could not find any downloads that satisfy the requirement blinker==1.3 (from -r requirements.txt (line 1))
  No distributions at all found for blinker==1.3 (from -r requirements.txt (line 1))

So for some reason pip can't access any packages from inside a docker container. Is there anything I need to do to allow it internet access?

However pip works fine to install things outside of the docker container, and worked fine even with that exact package (blinker==1.3) so that's not the problem. Also this problem isn't specific to that package. I get the same issue with any pip install command for any package.

Does anyone have any idea what's going on here?

Your problem comes from the fact that Docker is not using the proper DNS server. You can fix it in three different ways :

1. Adding Google DNS to your local config

Modifying /etc/resolv.conf and adding the following lines at the end

# Google IPv4 nameservers nameserver 8.8.8.8 nameserver 8.8.4.4

If you want to add other DNS servers, have a look here.

However this change won't be permanent (see this thread). To make it permanent : $ sudo nano /etc/dhcp/dhclient.conf Uncomment and edit the line with prepend domain-name-server : prepend domain-name-servers 8.8.8.8, 8.8.4.4;

Restart dhclient : $ sudo dhclient.

2. Modifying Docker config

As explained in the docs :

Systems that run Ubuntu or an Ubuntu derivative on the desktop typically use 127.0.0.1 as the default nameserver in /etc/resolv.conf file.

To specify a DNS server for use by Docker :

1. Log into Ubuntu as a user with sudo privileges.

2. Open the /etc/default/docker file for editing :

    $ sudo nano /etc/default/docker

3. Add the following setting for Docker.

    DOCKER_OPTS="--dns 8.8.8.8"

4. Save and close the file.

5. Restart the Docker daemon :

    $ sudo systemctl restart docker

3. Using a parameter when you run Docker

When you run docker, simply add the following parameter : --dns 8.8.8.8

I needed to add --network=host to my docker build command:

docker build --network=host -t image_name .

I had the same issue and it plagued me for a while and I tried a lot of solutions online but to no avail. However I finally resolved it as follows:

Running:

Ubuntu 16.04 
docker Server 18.03.0-ce

1. Discover the address of your DNS server.

   Discover the address of your DNS server by running the following command:

   $: nmcli dev show | grep 'IP4.DNS'
   IP4.DNS[1]:                192.168.210.2
   

2. Update the Docker daemon

   Create a docker config file at /etc/docker/daemon.json. (if you don't already have one) and add the following content to the file:

   {
       "dns": ["192.168.210.2", "8.8.8.8"]
   }
   

   The first item of the array is your network's DNS server and the second is google's DNS server as a fallback when if your network's DNS is not available.

   Save the file and then restart the docker service

   $: sudo service docker restart