Why are Smalltalk images considered bad but Docker images are great?

Because Docker lets you put arbitrary things into an image. Smalltalk images let you put arbitrary Smalltalk into an image.

It's apples and oranges: I can't mix-and-match technologies in Smalltalk images.

(I'd also argue that it isn't so much that Smalltalk images are considered "bad", rather that Smalltalk images don't suit the needs of a lot of people. Or problem spaces.)

Smalltalk expected the image to hold the authoritative copy of your source code, adding friction to the use of any tools that were not first ported to the Smalltalk you were using (e.g., your favorite editor, version control, code review).

Because docker is the new hotness while smalltalk is old hotness?

My understanding of Smalltalk is extremely small, but, this is an apple to well a tree, so smalltalk images and docker images are both ways to persist some sort of state at runtime, however, Smalltalk images are more like configuration while docker images are more of an entire environment.