dev/ci/README.md update (#44010)

dev/bots/deploy_gallery.sh

@@ -100,6 +100,9 @@ elif [[ "$OS" == "darwin" ]]; then
       echo "Testing archiving with distribution profile..."
       (
         cd examples/flutter_gallery/ios
+        # Cirrus Mac VMs come with an old version of fastlane which was causing
+        # dependency issues (https://github.com/flutter/flutter/issues/43435),
+        # so explicitly use the version specified in $BUNDLE_GEMFILE.
         bundle exec fastlane build_and_deploy_testflight
       )
       echo "(Not deploying; Flutter Gallery is only deployed to TestFlight for tagged dev branch commits.)"

dev/ci/README.md

-This directory includes scripts and tools for continuous integration builds and tests.
+This directory includes scripts and tools for setting up Flutter's continuous
+integration environments.
+
+## Cirrus Linux
+
+Flutter's Linux tasks run on a custom Docker image. The `Dockerfile` for this
+image can be found at [/dev/ci/docker_linux/Dockerfile](https://github.com/flutter/flutter/blob/master/dev/ci/docker_linux/Dockerfile).
+On each new change to this `Dockerfile`, Cirrus will build a new version of
+the Docker image as a dependency to any Linux tests. It is no longer necessary
+to manually build and push the Docker image locally.
+
+NOTE: there are some factors external to the actual `Dockerfile` that would
+necessitate rebuilding the Docker image, such as upstream code changes, (Linux
+distribution) repository updates, or a file that gets `COPY`ied into the image
+changing. In this case, a trivial `Dockerfile` change (such as a comment)
+would invalidate the cache and trigger a rebuild.