Added a Dockerfile to build Ubuntu 18.04 based image with all the required dependencies to run mongo build.

Original Summary

Building mongodb source code with Docker container

Original Description

Building mongodb source code requires some specific versions of gcc/g++, python and other libraries. For new developers trying to build mongodb, it can be a lot of hassle to get all the correct versions installed.

It will be useful to have a pre-built docker image with all the dependencies with specific versions installed. The build can then be done using the docker container, saving time to setup developer machines.

With editors like VSCode providing nice docker integration, it will be easy for developers to try code changes quickly, without requiring a lot of setup.