Python標準のTemplateを使う

build.py

# encoding: UTF-8

from string import Template
import os

with open('src/layout.html') as template_file:
    s = Template(template_file.read())

for content_file in os.listdir('src/content'):
    with open('src/content/'+content_file) as input_file:
        c = input_file.read()
    with open('out/'+content_file,'w') as output_file:
        output_file.write(s.substitute(content=c))
print('exit')