Use shell to invoke ChordPro

This allows us to run the makefile with or without a shell function
with Docker. This should work both locally and during build in the
pipeline.

makefile

@@ -6,13 +6,15 @@ SONG_PDF := $(patsubst %$(SRC_EXTENSION),%.pdf,$(SONG_CHO))
 SONG_HTML := $(patsubst %$(SRC_EXTENSION),%.html,$(SONG_CHO))
 SONGBOOK := songbook.pdf
 
+SHELL := $(shell echo $$SHELL)
+
 .DEFAULT_GOAL := all
 
 %.pdf: %$(SRC_EXTENSION)
-	chordpro --config=chordpro.json -o $@ $<
+	$(SHELL) -c 'chordpro --config=chordpro.json -o $@ $<'
 
 %.html: %$(SRC_EXTENSION)
-	chordpro --config=chordpro.json -o $@ $<
+	$(SHELL)  -c 'chordpro --config=chordpro.json -o $@ $<'
 
 all: $(SONG_PDF) $(SONG_HTML) $(SONGBOOK)
 

readme.md

@@ -4,6 +4,26 @@
 - `ghostscript`
 - `zola` (optional)
 
+You can [install ChordPro][1] directly on your machine, or use Docker
+with the official iamge `chordpro/chordpro:latest`. It can be seamlesly
+integrated with a shell alias:
+
+```
+alias chordpro="docker run -ti --rm --env HOME=\$HOME --env USER=\$USER --workdir \$(pwd) --volume \$HOME:\$HOME chordpro/chordpro:latest chordpro"
+```
+
+This will not function with the included makefile, so you will have to
+export this as a shell function (for example in `.bashrc`):
+
+```
+chordpro() {
+	docker run -ti --rm --env HOME=$HOME --env USER=$USER --workdir "$(pwd)" --volume "$HOME:$HOME" chordpro/chordpro:latest chordpro "$@"
+}
+export -f chordpro
+```
+
+[1]: https://www.chordpro.org/chordpro/chordpro-installation/
+
 ## Building
 
 `make` defaults to `all` which builds individual PDF and HTML files for