Side note: PickNik trimmed 5 minutes off a 45-minute Docker build time by disabling man-db in a Dockerfile
# Disable man-db to speed up installation (skip "Processing triggers for man-db")
# https://unix.stackexchange.com/a/53050
# We use a pipe in this stage and want to make sure pipe failures are caught
SHELL [ "/bin/bash", "-o", "pipefail", "-c" ]
RUN echo 'man-db man-db/auto-update boolean false' | debconf-set-selections
I believe this is most important when lots of debians are being installed.