One of the handiest features of the docker build system is the caching system. 'docker build' tries to reuse the layers already built until something changes inside the Dockerfile. In this way, we can save several minutes when rebuilding an image if the changes happen further down the list in the Dockerfile. Sometimes, though, we do want to invalidate the cache and ensure the next build won't use it. To do this an option is to pass the '--no-cache' argument to 'docker build'. When dealing with 'apt-get install' instructions though there are other tricks. I found this document on Dockerfile best practices very useful. First of all an observation. If you have 'RUN apt-get update' as a single line of a Dockerfile, followed by the installation of a package, e.g.: RUN apt-get update RUN apt-get install -y nginx then changing the list of packages and running again the build command won't trigger an 'apt-get update': that...