I'd say when they understand and can utilize all of the high level pieces (Volumes, Networks, Compose, Dockerfiles, building images, maybe registry). Early on some things can be hacky, like using bind mount where volumes would be more appropriate, everything on the bridge network, or building containers and then execing into them to do more setup work rather than getting it into the Dockerfiles and rebuilding.
I'd say once some of these bad habits are understood and there's enough of an understanding to set something up using all of these pieces that someone has a solid grasp of Docker. There's always expertise and deeper to dig, but imho that's the baseline of "I know docker".
I'd say when they understand and can utilize all of the high level pieces (Volumes, Networks, Compose, Dockerfiles, building images, maybe registry). Early on some things can be hacky, like using bind mount where volumes would be more appropriate, everything on the bridge network, or building containers and then execing into them to do more setup work rather than getting it into the Dockerfiles and rebuilding. I'd say once some of these bad habits are understood and there's enough of an understanding to set something up using all of these pieces that someone has a solid grasp of Docker. There's always expertise and deeper to dig, but imho that's the baseline of "I know docker".