[doc] Document Docker clang-format version

Signed-off-by: Sam Elliott <selliott@lowrisc.org>
diff --git a/util/container/README.md b/util/container/README.md
index ed4afa0..cc3ad3c 100644
--- a/util/container/README.md
+++ b/util/container/README.md
@@ -7,8 +7,11 @@
 * fusesoc
 * OpenOCD
 * RISCV toolchain
+* clang-format
 * Verilator
 
+The versions of the above should match the versions installed in CI.
+
 ## Local Build Instructions
 
 Skip this step if planning to use the pre-built container. To build in local