Extends the default image instead of creating a fresh one (#380)

Extends the default image instead of creating a fresh one

Author: dschep

README.md

@@ -386,19 +386,10 @@ For usage of `dockerizePip` on Windows do Step 1 only if running serverless on w
 Some Python packages require extra OS dependencies to build successfully. To deal with this, replace the default image (`lambci/lambda:python3.6`) with a `Dockerfile` like:
 
 ```dockerfile
-# AWS Lambda execution environment is based on Amazon Linux 1
-FROM amazonlinux:1
-
-# Install Python 3.6
-RUN yum -y install python36 python36-pip
+FROM lambci/lambda:build-python3.6
 
 # Install your dependencies
-RUN curl -s https://bootstrap.pypa.io/get-pip.py | python3
-RUN yum -y install python3-devel mysql-devel gcc
-
-# Set the same WORKDIR as default image
-RUN mkdir /var/task
-WORKDIR /var/task
+RUN yum -y install mysql-devel
 ```
 
 Then update your `serverless.yml`: