Labels

Our first new directive here is LABEL:

LABEL version="1.0"
LABEL org.sgnn7.name="python-webserver"

LABEL <key>=<value> or LABEL <key> <value> is used to add metadata about the image that is being built, which can later be examined and filtered by docker ps and docker images using something like docker images --filter "<key>=<value>". Keys are generally all lowercase in the reverse-dns notation, but you can use anything you want here and version should be present on every image, so we use the top-level version key name. However, the version here is not only there so that we can filter images but also to break Docker's cache if we change it. Without cache-busting of this sort or through the manually set flag during builds ( docker build --no-cache ), Docker will keep reusing the cache all the way up to the most recently changed directive or files so there is a high probability that your container will stay stuck in a frozen package configuration. This condition may or may not be what you want, but just in case you have automated build tooling, adding a version layer that can break the cache whenever you change it makes the container very easy to update.