diff --git a/Dockerfile b/Dockerfile
index 2e16d7b..0fe562f 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -26,6 +26,8 @@ EOT
 
 FROM base
 
+ENV PORT=4567
+
 # RUN useradd ruby --home /app --shell /bin/sh
 RUN useradd --home /app --create-home app
 
@@ -37,4 +39,4 @@ COPY --from=build --chown=app:app /app /app
 COPY --chown=app:app . .
 
 EXPOSE 4567
-CMD [ "bundle", "exec", "rackup", "--host", "0.0.0.0", "--port", "4567" ]
+CMD [ "bundle", "exec", "rackup", "--host", "0.0.0.0", "--port", "$PORT" ]