make_deb.sh invalid invokation in `make ubuntu`

[gvfnix]

Hi. When I run commands from build-linux/howto.md, make ubuntu command fails:

$ unzip ~/Downloads/pdagent-master.zip
$ cd pdagent-master/
$ gpg --homedir=build-linux/gnupg --gen-key
$ make ubuntu
...
Successfully tagged pdagent-ubuntu:latest
docker run \
	-v `pwd`:/usr/share/pdagent \
	-it pdagent-ubuntu \
		/bin/sh -c "/bin/sh build-linux/make_deb.sh /usr/share/pdagent/build-linux/gpg-deb /usr/share/pdagent/target"
Usage: build-linux/make_deb.sh {path-to-gpg-home} {path-to-package-installation-root}
Makefile:34: recipe for target 'target/deb' failed
make: *** [target/deb] Error 2

I believe that this happens due to incorrect naming of gpg-home directory. While gpg setup offers creating of build-linux/gnupg directory, make ubuntu uses build-linux/gpg-deb. So I presume that build-linux/howto.md should be edited.