Add instructions to build a docker image for pgloader.

The current instructions build SBCL then pgloader all from sources in a
debian VM. It might be a good idea to maintain a debian+SBCL docker
image and build pgloader on top of that.

INSTALL.md

@@ -83,3 +83,68 @@ image, at compile time. This defaults to 4 GB.
     make DYNSIZE=1024
     
 Now the `./build/bin/pgloader` that you get only uses 1GB.
+
+## Building a docker image
+
+We start with a `debian` image:
+
+	docker run -it debian bash
+
+And then run the following steps:
+
+    # apt-get update
+    # apt-get install -y wget curl make git bzip2 time libzip-dev openssl-dev
+    # apt-get install -y patch unzip libsqlite3-dev gawk freetds-dev
+    # useradd -m -s /bin/bash dim
+    # su - dim
+    
+Install a binary version on SBCL, which unfortunately has no support for
+core compression, so only use it to build another SBCL version from sources
+with proper options:
+
+    $ mkdir sbcl
+    $ cd sbcl
+    $ wget http://prdownloads.sourceforge.net/sbcl/sbcl-1.2.6-x86-64-linux-binary.tar.bz2
+    $ wget http://prdownloads.sourceforge.net/sbcl/sbcl-1.2.6-source.tar.bz2?download
+    $ mv sbcl-1.2.6-source.tar.bz2\?download sbcl-1.2.6-source.tar.bz2
+    $ tar xf sbcl-1.2.6-x86-64-linux-binary.tar.bz2
+    $ tar xf sbcl-1.2.6-source.tar.bz2
+    $ exit
+    
+Install SBCL as root
+
+    # cd /home/dim/sbcl/sbcl-1.2.6-x86-64-linux
+    # bash install.sh
+
+Now back as the unprivileged user (dim) to compile SBCL from sources:
+
+    # su - dim
+    $ cd sbcl/sbcl-1.2.6
+    $ sh make.sh --with-sb-core-compression --with-sb-thread > build.out 2>&1
+    $ exit
+
+And install the newly compiled SBCL as root:
+
+    # cd /home/dim/sbcl/sbcl-1.2.6
+    # sh install.sh
+    
+Now build pgloader from sources:
+
+    # su - dim
+    $ git clone https://github.com/dimitri/pgloader
+    $ cd pgloader
+    $ make
+    $ ./build/bin/pgloader --help
+    $ exit
+
+Now install pgloader in `/usr/local/bin` to make it easy to use:
+
+    # cp /home/dim/pgloader/build/bin/pgloader /usr/local/bin
+    # pgloader --version
+
+Commit the docker instance and push it, from the host:
+
+    $ docker login
+    $ docker ps -l
+    $ docker commit <id> dimitri/pgloader-3.1.cd52654
+    $ docker push dimitri/pgloader-3.1.cd52654