[docker] Read the digest from imagetools output instead of --format (project-chip#73587)

The buildx in CI ignores --format and prints the whole human readable record,
so remote_digest returned that record rather than a digest. It still compares
as non-empty, so skipping the version tag kept working, but comparing two of
them never matches because each names its own tag, and latest was therefore
pushed on every run.

Parse the Digest line, which the unformatted output has carried across
versions, so the comparison works whatever buildx is present.

Author: dcj

integrations/docker/build.sh

@@ -79,7 +79,12 @@ die() {
 # Always succeeds: an unknown tag is an answer, not an error, and the callers
 # run under set -e where a non-zero status would abort the script.
 remote_digest() {
-    docker buildx imagetools inspect --format '{{.Manifest.Digest}}' "$1" 2>/dev/null || true
+    # Parse the Digest line rather than asking for it with --format: older
+    # buildx ignores the flag and prints the whole human readable record, which
+    # still compares as non-empty and so silently defeats any comparison
+    # between two of these. The unformatted output has carried a "Digest:" line
+    # across versions.
+    docker buildx imagetools inspect "$1" 2>/dev/null | awk '$1 == "Digest:" { print $2; exit }'
 }
 
 set -ex