diff --git a/.changes/pdm-build.fixed.md b/.changes/pdm-build.fixed.md
new file mode 100644
index 0000000..86d481a
--- /dev/null
+++ b/.changes/pdm-build.fixed.md
@@ -0,0 +1 @@
+Keep runtime image builds compatible with PDM 2.28 by pinning its unearth installer dependency to 0.18.3.
diff --git a/Dockerfile b/Dockerfile
index 7de4230..8f209f1 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -11,7 +11,8 @@ ENV PDM_CHECK_UPDATE=false \
     PDM_VENV_IN_PROJECT=1
 WORKDIR /app
 
-RUN pip install --no-cache-dir "pdm==${PDM_VERSION}"
+# PDM 2.28 requires the httpx client replaced in unearth 0.19.
+RUN pip install --no-cache-dir "pdm==${PDM_VERSION}" "unearth==0.18.3"
 
 COPY pyproject.toml pdm.lock README.md ./
 RUN pdm install --prod --no-editable --frozen-lockfile