Use docker for testing

[carltongibson]

Having played with Travis for Python 3.6 support, and not wanting to duplicate the tox config if possible, I thought to try using a preconfigured image with docker.

Not sure whether to merge this. A little slow-down would OK — I really don't want to duplicate the build matrix — that is a pain — but it would be nice to do better that 2x

• Re-add coverage
• Investigate caching first image fetch (≈50secs.)
• Why no time difference between tox build and detox build? (cp 1065 vs 1066 — diff of 2s)
• Why slower than before (even allowing for image fetch)? Build 1024 2m10s vs Build 1066 5m34s? — so if we could cache the image fetch we're looking at 2mins vs 4...

[carltongibson]

There is at least one extra env here (py36-djangolatest-restframeworklatest) so that's a few seconds.

[carltongibson]

Main issue is pip caching. Should be able to resolve that with a mount.

[carltongibson]

It looks like this will remain slower (at least for now): 3f6fb0b tries installing the requirements at the docker build step. This make ≈0 difference to the build time (maybe a couple of seconds). The subsequent docker run step is not affected (it still takes ages™)

Cf Build 1073 and Build 1066

I'm guessing the bottom line is that Docker (or the host VM) is underpowered — particularly with regard to parallelisation — compared to the travis' own containers. (Just guessing.)

Also there's a spurious error on the warnings build:

  File "/src/.tox/warnings/lib/python3.6/site-packages/django/db/backends/utils.py", line 187
    match = re.match('([^"]+)"\."([^"]+)', name)
                        ^
SyntaxError: invalid escape sequence \.

I'll keep playing with this but it's not going to go in just yet.

[carltongibson]

@rpkilby I see you contributed to tox-travis — does that package solve the duplication issue? (It looks like it should.)

[carltongibson]

It turns out that "Yes" is the answer there. #651

[rpkilby]

Yep - I started to submit a PR, but was hesitant since I was duplicating the build matrix in both tox and travis. That would be similar to DRF's tox config right now. I hand't thought to use tox-travis to just run all of the python version specific builds.

[carltongibson]

For me the prime concern is keeping the matrix in one place. tox-travis does that. So (currently) it's favourite. 🙂

[rpkilby]

Yeah - the duplication isn't as bad, but it's still annoying to figure out what to correctly include/exclude in the travis matrix. Your approach is a good compromise