Fix typo in docker/Makefile (#216)

diff --git a/docker/Makefile b/docker/Makefile
index 3fb5e2c..0c46f33 100644
--- a/docker/Makefile
+++ b/docker/Makefile
@@ -24,7 +24,7 @@
 py3.7: BASE_PYTHON_IMAGE = python:3.7
 py3.8: BASE_PYTHON_IMAGE = python:3.8
 py3.9: BASE_PYTHON_IMAGE = python:3.9
-py3.9: BASE_PYTHON_IMAGE = python:3.10
+py3.10: BASE_PYTHON_IMAGE = python:3.10
 
 PUSH_TARGETS := $(TARGETS:%=push-%)