diff --git a/Dockerfile b/Dockerfile
index 7caf20b..11cd1ee 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -90,6 +90,11 @@ ENV DOTNET_CLI_TELEMETRY_OPTOUT=1 \
     DOTNET_NOLOGO=1
 
 # ============================================================
+# Switch to bash for the bash-ism layers below (${var//pattern} etc).
+# Layers above (apt base / Node / Go / .NET) only use POSIX so they cached fine
+# under dash; SHELL is set here to invalidate the cache for layer 6+ only.
+SHELL ["/bin/bash", "-c"]
+
 # 5. Swift (Ubuntu 22.04 tarball — works on Debian bookworm
 #    because bookworm ships the right libicu/libstdc++ baseline)
 # ============================================================