multistage build: cannot reference alias from previous stage(s)

[j-ogas]

Reproducer:

FROM alpine as a
RUN echo foo

FROM a as b
RUN echo bar

FROM b
RUN echo qux

Expected behavior: image builds without issue.

Actual behavior:

$ ch-image build -t multi .
  1. FROM alpine AS a
[...]
  4. FROM a AS b
error: unauthorized or not in registry: registry-1.docker.io:443/library/a:latest
hint: if your registry needs authentication, use --auth

It appears to fail for one or more of the following reasons: (1) the tag has :latest appended by default, (2) ch-image does store images in storage with their alias, i.e., there is no a, b, or c in storage; rather there is multi_stage0, etc., and (3) ch-image doesn't check if the FROM argument matches an previous stage image alias.

[reidpr]

Closed by PR #1581.