man: build dockerd man pages using make

Vendor the go-md2man tool used to generate the man pages so that the
only dependency is a Go toolchain.

Signed-off-by: Cory Snider <csnider@mirantis.com>

man/Makefile

@@ -0,0 +1,41 @@
+prefix = /usr/local
+mandir = $(prefix)/man
+INSTALL = install
+INSTALL_DATA = ${INSTALL} -m 644
+
+ALL_PAGES := $(wildcard *.*.md)
+
+# Determine which manual sections we are generating pages for
+# by isolating the last part of the filename before the extension
+# and eliminating duplicates.
+man_section = $(lastword $(subst ., ,$(1)))
+sections := $(sort $(foreach page,$(ALL_PAGES:.md=),$(call man_section,$(page))))
+
+# Dynamically generate pattern rules for each manual section
+# so make knows how to build a target like man8/dockerd.8.
+define MANPAGE_template
+man$(1)/%.$(1): %.$(1).md .build/go-md2man | man$(1)
+	.build/go-md2man -in $$< -out $$@
+endef
+$(foreach sec,$(sections),$(eval $(call MANPAGE_template,$(sec))))
+
+# Default target: build all man pages.
+all: $(foreach page,$(ALL_PAGES:.md=),man$(call man_section,$(page))/$(page))
+
+# Target for creating the man{1..8} directories as needed.
+.PRECIOUS: man%
+man%:
+	-mkdir $@
+
+.PHONY: install
+install: all
+	for sec in $(sections); do \
+		$(INSTALL_DATA) man$$sec/* $(DESTDIR)$(mandir)/man$$sec; \
+	done
+
+.build/go-md2man: go.mod go.sum
+	GO111MODULE=auto go build -o $@ github.com/cpuguy83/go-md2man/v2
+
+.PHONY: clean
+clean:
+	rm -r man* .build