Added Docker support (#40)

* Added Docker support

- replaced necessity for devicemapper (through kpartx) by using parted and
  losetup with offsets
- added Dockerfile
- added dependency for parted and grep
- added hints to README.md
- common: loop through unmounts, fix shellcheck warnings

* stage2: use debconf instead of console-setup patch. Fixes #41

Dockerfile

@@ -0,0 +1,12 @@
+FROM debian:jessie
+
+RUN apt-get -y update && \
+    apt-get -y install \
+        git vim parted \
+        quilt realpath qemu-user-static debootstrap zerofree pxz zip dosfstools \
+        bsdtar libcap2-bin rsync grep \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY . /pi-gen/
+
+VOLUME [ "/pi-gen/work", "/pi-gen/deploy"]