Switch to Docker-only builds

Makefile

-
-# Which Roundcube version to install.
-RC_VERSION = 1.4.4
-
-TARGET = ./build
-
-all: build
-
-build:
-	mkdir -p $(TARGET)
-	./install-roundcube --version=$(RC_VERSION) --destdir=$(TARGET)/src $@
-	echo $(RC_VERSION) > $(TARGET)/src/.rc_version
-
-clean:
-	-rm -fr $(TARGET)
-
-.PHONY: build clean
-