fix: restore bin/docker-build functionality

[cratelyn]

this branch makes changes to scripts in the bin/ directory, to
restore the end-to-end local development workflow described in
BUILD.md here:
https://github.com/linkerd/linkerd2/blob/main/BUILD.md#comprehensive

now that we no longer include a bin-cli in our published artifacts,
we can remove that image from the list of images that the _docker.sh
helper script is aware of. this means we now refrain from (a)
building the image when bin/docker-build is run, and (b) loading the
image when bin/image-load is run.

some related comments are updated in this branch, while we are here.

---

• refactor: remove cli-bin from _docker.sh images
  this helper script wrapping docker is used by various scripts in the
  development workflow.

  in chore(ci)!: update CLI build pipeline  #14360, we removed the
  cli-bin image from our published artifacts.

  Signed-off-by: katelyn martin [email protected]

• nit(bin): update build-cli-bin comment
  this commit removes mention of docker from this comment. there is no
  longer an equivalent docker-build-cli-bin script, so this is the way
  that we build the cli binary.

  Signed-off-by: katelyn martin [email protected]

• fix(bin): remove cli from bin/docker-build
  this commit removes mention of the cli from our bin/docker-build
  script.

  in chore(ci)!: update CLI build pipeline  #14360, we removed the cli
  image from our published artifacts. moreover, invoking bin/linkerd
  will call bin/build-cli-bin.

  so, this commit removes both calls, to the now deleted
  bin/docker-build-cli-bin script, as well as the bin/build-cli-bin
  script. this lets the bin/docker-build script focus solely on docker
  images.

  Signed-off-by: katelyn martin [email protected]

• docs(cli): a comment about cli image ENTRYPOINT
  this image is used within CI, and broadly as a "layer" to place the cli
  binaries in a final image.

  the cli image itself however, is not currently intended for use directly
  via docker build, and will consequently yield an error response if
  built directly.

  this commit adds a comment providing some information about this.

  see chore(ci)!: update CLI build pipeline  #14360 for more.

  Signed-off-by: katelyn martin [email protected]