Fixed absolute links in external content

[aduermael]

Proposed changes

The files we’re pulling from docker/docker may include links to docs.docker.com. And we can’t forbid that because relative links wouldn’t make sense in the context of docker/docker repository in some situations. So let’s just fix these links right after they get imported.

This is necessary for tests in #849 to pass.

[aduermael]

ping @johndmulhausen

[thaJeztah]

Adding the obligatory link to the "don't use regex to parse HTML" post http://stackoverflow.com/a/1732454/1811501 here 😇

[thaJeztah]

But yeah, I guess this works; when thinking about this, I wondered if we could simply strip https://docs.docker.com/ in case there's links somehow that used single quotes, but I assume that's not a problem here

[aduermael]

hehe, yes, I don't like doing this too much...

[johndmulhausen]

Nobody likes doing this - it's just the worst. :/

[aduermael]

@johndmulhausen can you merge that one please if you don't have comments?

These links are the only reason why this new test fails: #849

18:55:52 --- FAIL: TestURLs (2.11s)
18:55:52 	html_test.go:53: found absolute link: <a href="https://docs.docker.com/engine/admin/logging/overview/"> - /engine/reference/commandline/logs/index.html
18:55:52 	html_test.go:53: found absolute link: <a href="https://docs.docker.com/engine/tutorials/dockervolumes/"> - /engine/reference/commandline/run/index.html
18:55:52 	html_test.go:53: found absolute link: <a href="https://docs.docker.com/engine/swarm/ingress/"> - /engine/reference/commandline/service_create/index.html

I would like to move forward and keep fixing content in docker/docker.github.io. If we don't need these 2 lines in a near future, we can simply remove them.

Thanks!

[thaJeztah]

LGTM

[aduermael]

thanks @thaJeztah !

[johndmulhausen]

Thanks, @aduermael :)