Add note to enable BuildKit to build in docker

[zappy-shu]

- What I did

Adds a note in the "Build using Docker" section of BUILDING.md to explain that BuildKit is required and how to enable it.

- A picture of a cute animal (not mandatory)

[ndeloof]

@zappy-shu build failed by jenkins internal issue, can your please git ci --amend your last commit and force push to trigger a fresh new build ?

[codecov]

Codecov Report

Merging #729 into master will increase coverage by 0.59%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #729      +/-   ##
==========================================
+ Coverage   70.94%   71.54%   +0.59%     
==========================================
  Files          61       60       -1     
  Lines        3493     3201     -292     
==========================================
- Hits         2478     2290     -188     
+ Misses        695      603      -92     
+ Partials      320      308      -12

Impacted Files	Coverage Δ
internal/packager/init.go	61.68% <0%> (-9.13%)	⬇️
internal/commands/root.go	73.86% <0%> (-4.65%)	⬇️
internal/packager/cnab.go	97.91% <0%> (-0.09%)	⬇️
internal/names.go	100% <0%> (ø)	⬆️
internal/commands/inspect.go
types/parameters/parameters.go	93.65% <0%> (+1.58%)	⬆️
internal/inspect/inspect.go	86.88% <0%> (+8.09%)	⬆️

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