Clean up the Makefile.

Dockerfile

@@ -43,4 +43,4 @@ WORKDIR /volume
 ENV HOME /volume
 # Run a test build.
 COPY example/ /example/
-RUN make -C /example all clean
+RUN make --debug=j --keep-going -C /example test

example/Makefile

-.PHONY: all clean
+.PHONY: test
+test: clean all
 
+.PHONY: all
 all: presentation.pdf left-to-right.pdf
 
 presentation.pdf: diagram.pdf
@@ -17,5 +19,6 @@ left-to-right.pdf: left-to-right.md
 %.gif: %.tty
 	ttyrec2gif -in $< -out $@
 
+.PHONY: clean
 clean:
 	- rm *.pdf *.gif