builder: add df command

[tiborvass]

Depends on #1327

Adds docker builder df to show only build cache usage.

[vdemeester]

LGTM 🐯

[codecov-io]

Codecov Report

Merging #1333 into master will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@## master #1333 +/- ##
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Coverage 54.64% 54.64% =======================================
Files 293 293 Lines 19360 19360 =======================================
Hits 10580 10580 Misses 8119 8119 Partials 661 661

[andrewhsu]

SGTM

[AkihiroSuda]

why was this closed?

[tiborvass]

@AkihiroSuda Because I'm working on another PR to open soon to show all detailed information in system df by removing the conflict between -v and --format, so you could do docker system df -v --format raw or docker system df -v --format '{{json .}}' There were a couple of issues with builder df: it was calling /system/df, no way of filtering with options. We'd rather do it right then rush it in.