Use Docker to invoke ChordPro

This allows us to run Make in a predictable environemnt without having
to install all the perl dependencies. It should work both locally
and during build in the pipeline.

makefile

@@ -1,3 +1,9 @@
+ifdef CHORDPRO_PIPELINE
+	CHORDPRO_CMD = chordpro
+else
+	CHORDPRO_CMD = docker run -ti --rm --env HOME=$(HOME) --env USER=$(USER) --workdir $(shell pwd) --volume $(HOME):$(HOME) chordpro/chordpro:latest chordpro --config=chordpro.json 
+endif
+
 SRC_DIR := $(wildcard content/*)
 SRC_EXTENSION := .cho
 
@@ -9,10 +15,10 @@ SONGBOOK := songbook.pdf
 .DEFAULT_GOAL := all
 
 %.pdf: %$(SRC_EXTENSION)
-	chordpro --config=chordpro.json -o $@ $<
+	$(CHORDPRO_CMD) -o $@ $<
 
 %.html: %$(SRC_EXTENSION)
-	chordpro --config=chordpro.json -o $@ $<
+	$(CHORDPRO_CMD) -o $@ $<
 
 all: $(SONG_PDF) $(SONG_HTML) $(SONGBOOK)