Local development Docker improvements

[hiddewie]

When building the local development Docker image, a huge context is sent to the Docker daemon. Even worse, the whole image is usually rebuilt instead of Docker layer caching reusing the previous builds of the same image.

Example output

~/C/G/symfony-docs (request-locales|✔) $ docker build . -t symfony-docs
Sending build context to Docker daemon  164.4MB
Step 1/8 : FROM  python:2-stretch as builder
2-stretch: Pulling from library/python
7568c21980bd: Pull complete
4a9f2207c812: Pull complete
6fe350d2b140: Pull complete
d95a2fdc8b3d: Pull complete

# ...

The problem is 2 things

• a locally built _build directory is sent with the Docker context
• The .git directory is sent with the Docker context
  Both are not needed in the build process.

This has been fixed with a .dockerignore file, see the documentation here: https://docs.docker.com/engine/reference/builder/#dockerignore-file

New output:

~/C/G/symfony-docs (docker-performance|✔) $ docker build . -t symfony-docs
Sending build context to Docker daemon  12.88MB
Step 1/9 : FROM python:2-alpine as builder
2-alpine: Pulling from library/python
aad63a933944: Already exists
259d822268fb: Already exists

# ...

Next, the much smaller alpine variants of the base images can be used, which is smaller to download. The Python 2 and Nginx base image both have an alpine variant.

[javiereguiluz]

Thanks Hidde!

Please note that we're going to change the doc building completely soon (and move from Python to a PHP-based solution). But, let's improve this meanwhile.