docker: What is the default WORKDIR in a Dockerfile?

We can use WORKDIR to set a directory as work directory in Dockerfile, but what's the default value if we don't set?

docker workdir

says it is /, so the root directory

The default is indeed / as stated elsewhere. It is worth mentioning, though, that you will almost never be running from an empty docker image (FROM scratch), so the WORKDIR is likely set by the base image you're using.

For instance, https://github.com/docker-library/tomcat/blob/master/Dockerfile-alpine.template has WORKDIR $CATALINA_HOME, and https://github.com/dockerfile/ubuntu/blob/master/Dockerfile has WORKDIR /root. (https://hub.docker.com/r/base/archlinux/~/dockerfile/ does not use WORKDIR however.)

It is best, therefore, to set your own WORKDIR explicitly.

The default working directory for running binaries within a container is the root directory (/), but the developer can set a different directory with the Dockerfile WORKDIR command. The operator can override this with:

-w="": Working directory inside the container

Here: https://docs.docker.com/engine/reference/run/#workdir

As there is are no users but a root in a just born docker or a container strictly speaking. So, for every change there will be a commit for that container as this guy said. Hence the pwd of that or as you asked the WORKDIR is / root by default and every time you exec a /bin/bash like this:

$docker exec -i -t 53f784fwer54 /bin/bash

on a running container it will put you here / in the root dir.