Fix docs about docker build context

[shinsenter]

Docker's build context in this example should be placed in a sub directory, lets say ./.docker/ to reduce context size for building custom images, unless you want to copy all your files in webroot folder to your build context.

Including too many assets in the build context can become a performance drain.

https://github.com/serversideup/docker-php/blob/d76db022c596ddeb1316496909bb0012720ad1ac/README.md?plain=1#L285

[shinsenter]

Ref: https://docs.docker.com/engine/reference/commandline/build/

[jaydrogers]

Thanks, I will update the docs soon.

[jaydrogers]

Closing this issue because it's no longer relevant in the new docs. I appreciate your help on bringing this to our attention though!