Update "munge-i386" to add "--build-context" in "docker buildx build" lines

[tianon]

Without this, we end up building against the wrong image due to a bug in buildx/buildkit (docker/buildx#1886). 😞

[tianon]

(Because of this, we haven't noticed that RabbitMQ is failing to build on i386 and failing with a similar problem on arm32v7 😭)

[codecov-commenter]

Codecov Report

Merging #80 (d3dede4) into master (2ff9004) will not change coverage.
Report is 7 commits behind head on master.
The diff coverage is n/a.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

@@           Coverage Diff           @@
##           master      #80   +/-   ##
=======================================
  Coverage   73.10%   73.10%           
=======================================
  Files           7        7           
  Lines         714      714           
=======================================
  Hits          522      522           
  Misses        162      162           
  Partials       30       30           

[tianon]

Oh, I guess we could/should maybe add --platform linux/386 also?

[yosifkit]

Oh, I guess we could/should maybe add --platform linux/386 also?

Agreed.

[tianon]

(oh, #81 is very related, although not exactly the same since munge-i386 is separate)

[yosifkit]

Added a commit to address the --platform flag by building upon the provided flag from #81. (So now 81 is necessary for all the functionality here).

[tianon]

More heinous every time we touch it 😀

LGTM