Docker Build, DNS and CNTLM (Corporate Proxy)
Was practicing the Docker Get Started Manual , and had to build the image: docker build -t friendlyhello . Got errors while trying to install Flask with pip , like in this Dockerfile line: RUN pip install - r requirements . txt Figured out it should be corporate proxy. I have CNTLM to help Ubuntu deal with TMG . Tried every variation of http_proxy=http://localhost:3128 . No luck. A few hours into the problem, found this thread . It nailed the problem: cannot use "localhost" becase the docker image will also have it's own "localhost". Replaced "localhost" by my desktop's IP and voilà: pip install worked perfectly. ENV http_proxy http://XX.XX.XX.XX:3128 ENV https_proxy http://XX.XX.XX.XX:3128 Likewise, if the Dockerfile has to resolve a DNS entry, remember to add your DNS server to docker deamon , otherwise the default DNS entry 8.8.8.8 will be set and the docker build will not recognize DN