Log line "Build finished" is not correct

[humitos]

In

readthedocs.org/readthedocs/doc_builder/environments.py

Lines 547 to 555 in 637a91e

	log.info(
	'Build finished',
	# TODO: move all of these attributes to ``log.bind`` if possible
	project_slug=self.project.slug if self.project else '',
	version_slug=self.version.slug if self.version else '',
	# TODO: add organization_slug here
	success=self.build.get('success') if self.build else '',
	length=self.build.get('length') if self.build else '',
	)

we are logging Build finished (inside __exit__), but that line is called up to 3 times per build because we are using the DockerBuildEnvironment (which inherit from BuildEnvironment) 3 times with a context manager:

1. when syncing the repository:

   readthedocs.org/readthedocs/projects/tasks.py

   Line 354 in 637a91e

   env_cls = DockerBuildEnvironment

2. when running the setup:

   readthedocs.org/readthedocs/projects/tasks.py

   Line 642 in 637a91e

   env_cls = DockerBuildEnvironment

3. when running the build:

   readthedocs.org/readthedocs/projects/tasks.py

   Line 642 in 637a91e

   env_cls = DockerBuildEnvironment

Note that we are using this log line in the New Relic dashboard and it's currently showing not accurate data.

Related to #3984