I did it a little differently. Here is my setup.
Visually this is the directory layout
~/Arduino/Testy_app/ <- sketch dir
/Testy_app.ino <- has a #include "foo.h"
/foo <- a git repo
/foo/foo.h
/foo/foo.cpp
Here is how I build:
~/Arduino/Testy_App/$ arduino-cli compile --library "/home/davis/Arduino/Testy_app/foo/" --fqbn arduino:samd:mkrwan1310 Testy_app
If you wish to be more elaborate and specify libs and src dirs, this also works
~/Arduino/Testy_app/ <- sketch dir
/Testy_app.ino <- has a #include "foo.h"
/lib <- a git repo
/lib/foo/src/foo.h
/lib/foo/src/foo.cpp
and the build method is:
~/Arduino/Testy_App/$ arduino-cli compile --library "/home/davis/Arduino/Testy_app/lib/foo/src" --fqbn arduino:samd:mkrwan1310 Testy_app
One more bit of tweaking needs to be done to include files from the lib dirs to main dir. If you need to do that, this is the work around:
~/Arduino/Testy_app/ <- sketch dir
| /Testy_app.ino <- has a #include
"foo.h"
/inc/Testy_app.h
/foo <- a git repo
/foo/foo.h
/foo/foo.cpp < has a "include testy_app.h"
Then do the compile like this
~/Arduino/Testy_App/$ arduino-cli compile \
--library "/home/davis/Arduino/Testy_app/inc" \
--library "/home/davis/Arduino/Testy_app/foo/src" \
--fqbn arduino:samd:mkrwan1310 Testy_app