Set default Buildkit output mode to plain

[toolmantim]

As mentioned in #256, if you use Buildkit the default output can create a bunch of erroneous log output groups.

This change sets the default Buildkit output to plain to avoid this.

Fixes #256

[toolmantim]

One thing I didn't consider, is that people still need to set DOCKER_BUILDKIT=1 environment variable?

Which means, either:

1. We could just add a buildkit: true option to the plugin, which sets both of the environment variables for them
2. We could leave it as-is, but only set BUILDKIT_PROGRESS conditionally if DOCKER_BUILKIT == "1"

I think I'd prefer version 1. What do you think @jayco & @niceking?

[niceking]

Hey @toolmantim! Totally keen to pair on this!!

[niceking]

One thing I didn't consider, is that people still need to set DOCKER_BUILDKIT=1 environment variable?

Which means, either:

1. We could just add a buildkit: true option to the plugin, which sets both of the environment variables for them
2. We could leave it as-is, but only set BUILDKIT_PROGRESS conditionally if DOCKER_BUILKIT == "1"

I think I'd prefer version 1. What do you think @jayco & @niceking?

I prefer 1 too! Nicer to be explicit rather than doing sneaky we-assumed-this-because-you-did-this things that we may forget about later!

[toolmantim]

Another thing to note, is that it turns out there's a little more to this than we thought — there's also parts of the output that we'll need to modify if buildkit is set to true, it's not just about setting the output mode to plain.

[toolmantim]

Turns out this doesn't actually solve the problem people were having after all, so let's close this for now.