fix: read the image id from a quiet build

lib/kitchen/docker/helpers/image_helper.rb

@@ -27,6 +27,9 @@ module ImageHelper
         include Kitchen::Docker::Helpers::CliHelper
         include Kitchen::Docker::Helpers::ContainerHelper
 
+        # An id on a line of its own, which is all `docker build -q` prints.
+        QUIET_BUILD_IMAGE_ID = /\A(sha256:[[:xdigit:]]{64})\z/
+
         # Pulls the built image's id out of `docker build` output.
         #
         # Scanned in reverse, and against several patterns, because the wording has
@@ -37,6 +40,13 @@ module ImageHelper
         # @raise [Kitchen::ActionFailed] if no id could be found
         def parse_image_id(output)
           output.split("\n").reverse_each do |line|
+            line = line.strip
+            # `docker build -q` prints the id and nothing else -- none of the
+            # wording below appears -- so a build with `build_options: -q` had
+            # no line any of these matched and failed with "Could not parse
+            # Docker build output for image ID" (#225).
+            return Regexp.last_match(1) if line.match(QUIET_BUILD_IMAGE_ID)
+
             if line =~ /writing image (sha256:[[:xdigit:]]{64})(?: \d*\.\ds)? done/i
               img_id = line[/writing image (sha256:[[:xdigit:]]{64})(?: \d*\.\ds)? done/i, 1]
               return img_id

spec/image_helper_spec.rb

@@ -16,6 +16,30 @@
 
 describe Kitchen::Docker::Helpers::ImageHelper do
   describe "#parse_image_id" do
+    # From #225. `docker build -q` prints the id on a line of its own and
+    # nothing else -- no "writing image", no "naming to", no "successfully
+    # built" -- so every pattern the parser had missed it, and a build with
+    # `build_options: -q` failed with "Could not parse Docker build output for
+    # image ID" rather than producing an instance.
+    context "with a quiet build" do
+      it "reads the id docker printed on its own" do
+        expect(helper.parse_image_id(DockerOutput::BUILD_QUIET))
+          .to eq DockerOutput::BUILD_QUIET_IMAGE_ID
+      end
+
+      it "reads it when docker also wrote a warning to stderr" do
+        expect(helper.parse_image_id("WARNING: something happened\n#{DockerOutput::BUILD_QUIET}"))
+          .to eq DockerOutput::BUILD_QUIET_IMAGE_ID
+      end
+
+      it "does not mistake a digest that is part of a longer line for the id" do
+        # Ordinary build output is full of "... sha256:... done" lines. Only a
+        # line that is nothing but a digest is the quiet form.
+        expect { helper.parse_image_id("#6 exporting manifest sha256:#{"a" * 64} done\n") }
+          .to raise_error(Kitchen::ActionFailed)
+      end
+    end
+
     # Docker has changed how it reports the built image's id several times, and
     # each change has broken this parser. Every format the driver claims to
     # support gets a case here, against output copied from a real build.

spec/support/docker_output.rb

@@ -81,6 +81,15 @@ module DockerOutput
 
   BUILD_LEGACY_IMAGE_ID = "1a2b3c4d5e6f".freeze
 
+  # `docker build -q` on Docker 29.7.2. Quiet mode prints the id and nothing
+  # else -- no step lines, no exporting lines, none of the wording the other
+  # fixtures here are built around. Reachable through the driver as
+  # `build_options: -q`.
+  BUILD_QUIET = "sha256:ab86ce908a36ffb7de411a72550e416a4d8c268570a0f7313c284c79344d6c0f\n".freeze
+
+  BUILD_QUIET_IMAGE_ID =
+    "sha256:ab86ce908a36ffb7de411a72550e416a4d8c268570a0f7313c284c79344d6c0f".freeze
+
   # `docker run -d`, the ordinary case: the id and nothing else.
   RUN_CONTAINER_ID = "b89e1e8b07664a1ee0bd09decb833146eaf9cc810a152950e3576a56745944db".freeze
   RUN_CLEAN = "#{RUN_CONTAINER_ID}\n".freeze