README: simplify docker build instructions

Run the build command directly in docker, using a shell function, instead
of opening a shell.  Forward the user/group IDs so the build artifacts belong
to the user, not root.

README.md

@@ -82,17 +82,19 @@ To build a Docker image:
 docker build -t seastar-dev docker/dev
 ```
 
-To launch a container:
+Create an shell function for building insider the container (bash syntax given):
 
-```    
-$ docker run -v $HOME/seastar/:/seastar -i -t seastar-dev /bin/bash
+```
+$ seabuild() { docker run -v $HOME/seastar/:/seastar -u $(id -u):$(id -g) -w /seastar -t seastar-dev "$@"; }
 ```
 
-Finally, to build seastar inside the container:
+(it is recommended to put this inside your .bashrc or similar)
+
+To build inside a container:
 
 ```    
-cd /seastar
-ninja-build
+$ seabuild ./configure.py
+$ seabuild ninja-build
 ```
 
 ### Building with a DPDK network backend