diff --git a/.github/workflows/formatters.yml b/.github/workflows/formatters.yml
index b114eea..18f0383 100644
--- a/.github/workflows/formatters.yml
+++ b/.github/workflows/formatters.yml
@@ -5,12 +5,12 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout pull request
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
         with:
           ref: ${{ github.event.pull_request.head.sha }}
-      - name: Fetch master
-        run: |
-          git fetch
+          # No shallow clone, we want to be able to compare PR branch
+          # to main.
+          fetch-depth: 0
       - name: Test changed formatters
         run: |
           set -euo pipefail
diff --git a/.github/workflows/lint.yml b/.github/workflows/lint.yml
index 923f719..dff8cf0 100644
--- a/.github/workflows/lint.yml
+++ b/.github/workflows/lint.yml
@@ -12,7 +12,7 @@ jobs:
         emacs_version: [26, 27, 28, "master"]
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
       - name: Run linters
         env:
           VERSION: ${{ matrix.emacs_version }}
diff --git a/Makefile b/Makefile
index b48264c..1be76b8 100644
--- a/Makefile
+++ b/Makefile
@@ -80,7 +80,11 @@ docker: ## Start a Docker shell; e.g. make docker VERSION=25.3
 
 .PHONY: fmt-build  # env vars: FORMATTERS, TAG
 fmt-build: ## Build a Docker image with formatters installed
-	@test/formatters/build-image.bash
+	@COMMON=0 test/formatters/build-image.bash
+
+.PHONY: fmt-build-common  # env var: TAG
+fmt-build-common: ## Build a Docker image with just the common base
+	@COMMON=1 test/formatters/build-image.bash
 
 .PHONY: fmt-docker  # env var: TAG
 fmt-docker: ## Start a Docker shell for testing formatters
diff --git a/test/formatters/Dockerfile b/test/formatters/Dockerfile
index 4807552..0098cfe 100644
--- a/test/formatters/Dockerfile
+++ b/test/formatters/Dockerfile
@@ -1,10 +1,15 @@
 # Ubuntu 20.04 LTS supported until April 2025
-FROM ubuntu:20.04
+FROM ubuntu:20.04 AS common
 
 WORKDIR /build
 COPY install-common.bash /build/
 RUN ./install-common.bash
 
+# Add an intermediate tag so that it is possible to execute debugging
+# code before formatter installation. This is necessary because with
+# the newer docker build based on buildkit, you can't access
+# intermediate image layers by default anymore.
+FROM common
 ARG FORMATTERS
 COPY install-formatters.bash /build/
 COPY installers /build/installers/
diff --git a/test/formatters/apheleia-ft.el b/test/formatters/apheleia-ft.el
index de4a377..38877f7 100755
--- a/test/formatters/apheleia-ft.el
+++ b/test/formatters/apheleia-ft.el
@@ -9,12 +9,26 @@
 
 (require 'cl-lib)
 (require 'map)
+(require 'subr-x)
 
 (defvar apheleia-ft--test-dir
   (file-name-directory
    (or load-file-name buffer-file-name))
   "Directory containing this module.")
 
+(defvar apheleia-ft--repo-dir
+  (expand-file-name (locate-dominating-file apheleia-ft--test-dir ".git"))
+  "Root directory of the Git repository.
+Guaranteed to be absolute and expanded.")
+
+(defun apheleia-ft--relative-truename (path)
+  "Given PATH relative to repo root, resolve symlinks.
+Return another path relative to repo root."
+  (string-remove-prefix
+   apheleia-ft--repo-dir
+   (file-truename
+    (expand-file-name path apheleia-ft--repo-dir))))
+
 (defun apheleia-ft--get-formatters (&optional all)
   "Return list of strings naming the formatters to run.
 This is determined by the environment variable FORMATTERS,
@@ -56,6 +70,44 @@ already in memory on the current branch."
       (goto-char (point-min))
       (read (current-buffer)))))
 
+(defun apheleia-ft--files-changed-since (ref)
+  "Get a list of the files changed between REF and HEAD."
+  (let ((stderr-file (make-temp-file "apheleia-ft-stderr-")))
+    (with-temp-buffer
+      (let ((exit-status
+             (call-process
+              "git" nil (list (current-buffer) stderr-file) nil
+              "diff" "--name-only" "--diff-filter=d" (format "%s..." ref))))
+        (unless (zerop exit-status)
+          (with-temp-buffer
+            (insert-file-contents stderr-file)
+            (princ (buffer-string)))
+          (error "Failed to 'git diff', got exit status %S" exit-status)))
+      (split-string (buffer-string)))))
+
+(defun apheleia-ft--formatters-depending-on-file (changed-file)
+  "Given CHANGED-FILE, return list of formatters affected by it.
+Return formatters as string names. This is used to determine
+which formatters need tests to be run. CHANGED-FILE should be
+relative to repo root, as returned by git diff --name-only."
+  (setq changed-file (apheleia-ft--relative-truename changed-file))
+  (save-match-data
+    (cond
+     ((string-match
+       "^test/formatters/installers/\\([^/]+\\)\\.bash$" changed-file)
+      (list (match-string 1 changed-file)))
+     ((string-match
+       "^test/formatters/samplecode/\\([^/]+\\)/[^/]+$" changed-file)
+      (list (match-string 1 changed-file)))
+     ((string-match
+       "^scripts/formatters/\\([^/]+\\)$" changed-file)
+      (let ((script (match-string 1 changed-file)))
+        (map-keys
+         (map-filter
+          (lambda (fmt def)
+            (member script def))
+          apheleia-formatters)))))))
+
 (defun apheleia-ft--get-formatters-for-pull-request ()
   "Return list of formatter string names that were touched in this PR.
 This means their commands in `apheleia-formatters' are different
@@ -69,6 +121,13 @@ main."
        (unless (equal command (alist-get formatter old-formatters))
          (push (symbol-name formatter) touched-formatters)))
      new-formatters)
+    (mapc
+     (lambda (changed-file)
+       (setq touched-formatters
+             (nconc
+              (apheleia-ft--formatters-depending-on-file changed-file)
+              touched-formatters)))
+     (apheleia-ft--files-changed-since "origin/main"))
     touched-formatters))
 
 (defun apheleia-ft-changed ()
diff --git a/test/formatters/build-image.bash b/test/formatters/build-image.bash
index a118d12..2753ba9 100755
--- a/test/formatters/build-image.bash
+++ b/test/formatters/build-image.bash
@@ -4,7 +4,9 @@ set -euo pipefail
 
 echo >&2 "build-image.bash: tagging apheleia-formatters:${TAG:-latest}"
 
-if [[ -n "${FORMATTERS:-}" ]]; then
+if [[ "${COMMON}" == "1" ]]; then
+    echo "build-image.bash: will tag common base image only"
+elif [[ -n "${FORMATTERS:-}" ]]; then
     echo "build-image.bash: will install these formatters: ${FORMATTERS}"
 else
     echo "build-image.bash: will install all formatters by default"
@@ -17,6 +19,11 @@ if [[ "$OSTYPE" != darwin* ]] && [[ "$EUID" != 0 ]]; then
     docker=(sudo -E "${docker[@]}")
 fi
 
-exec "${docker[@]}" build .                    \
+args=()
+if [[ "${COMMON}" == "1" ]]; then
+    args+=(--target=common)
+fi
+
+exec "${docker[@]}" build . "${args[@]}"       \
      -t "apheleia-formatters:${TAG:-latest}"   \
      --build-arg "FORMATTERS=${FORMATTERS:-}"
diff --git a/test/formatters/installers/brittany.bash b/test/formatters/installers/brittany.bash
index 118870b..719e198 100644
--- a/test/formatters/installers/brittany.bash
+++ b/test/formatters/installers/brittany.bash
@@ -7,5 +7,5 @@ apt-get install -y cabal-install-3.4 ghc
 ln -s /opt/cabal/bin/cabal /usr/local/bin/
 
 cabal v2-update
-cabal v2-install brittany
+cabal v2-install brittany --reorder-goals
 cp -L "$HOME/.cabal/bin/brittany" /usr/local/bin/
diff --git a/test/formatters/run-func.bash b/test/formatters/run-func.bash
index 9eff543..621edde 100755
--- a/test/formatters/run-func.bash
+++ b/test/formatters/run-func.bash
@@ -8,4 +8,5 @@ set -euo pipefail
 cd "$(dirname "$0")"
 repo="$(cd ../.. && pwd)"
 
-exec emacs --batch -L "${repo}" -L . -l apheleia-ft -f "$1"
+exec emacs --batch -L "${repo}" -L . -l apheleia-ft \
+     --eval "(setq debug-on-error t)" -f "$1"