Pass MAKEFLAGS to docker make command

Reduces build time by 1/3 or so.

build/Makefile

@@ -25,7 +25,7 @@ all: $(TARGETS)
 # Build with preinstalled docker container; first install it with:
 #   docker pull riscvintl/riscv-docs-base-container-image:latest
 docker:
-	cd .. && docker run -it -v .:/build riscvintl/riscv-docs-base-container-image:latest /bin/sh -c 'cd ./build; make'
+	cd .. && docker run -it -v .:/build riscvintl/riscv-docs-base-container-image:latest /bin/sh -c 'cd ./build; make $(MAKEFLAGS)'
 
 # Asciidoctor options
 ASCIIDOCTOR_OPTS := -a compress \