ci: Automatically rebuild development/CI Docker images

[kostko]

Fixes #295

[codecov]

Codecov Report

Merging #2970 into master will increase coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2970      +/-   ##
==========================================
+ Coverage   68.41%   68.55%   +0.13%     
==========================================
  Files         360      360              
  Lines       35273    35273              
==========================================
+ Hits        24132    24180      +48     
+ Misses       8033     7997      -36     
+ Partials     3108     3096      -12     

Impacted Files	Coverage Δ
go/oasis-node/cmd/common/metrics/disk.go	65.51% <0.00%> (-20.69%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	84.00% <0.00%> (-8.00%)	⬇️
go/storage/mkvs/lookup.go	73.46% <0.00%> (-6.13%)	⬇️
go/consensus/tendermint/roothash/roothash.go	63.17% <0.00%> (-5.41%)	⬇️
...consensus/tendermint/apps/registry/transactions.go	49.72% <0.00%> (-2.19%)	⬇️
go/worker/common/p2p/p2p.go	66.21% <0.00%> (-1.81%)	⬇️
go/consensus/tendermint/tendermint.go	63.88% <0.00%> (-0.72%)	⬇️
go/roothash/tests/tester.go	95.58% <0.00%> (-0.64%)	⬇️
go/roothash/api/commitment/pool.go	69.38% <0.00%> (-0.57%)	⬇️
.../consensus/tendermint/apps/registry/state/state.go	58.61% <0.00%> (ø)
... and 19 more

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[kostko]

The Buildkite CI will fail as it needs a pipeline configuration change that will break all other jobs, so I haven't made it yet. Once approved I'll change Buildkite config and merge this.

[tjanez]

Good job!

[tjanez]

I think this triggering configuration means that Docker images for stable/* branches will only be built the first time a new stable/* branch is pushed to GitHub and never again, unless there is a manual change to something in the docker/ directory.

In my opinion, this is fine for now, but perhaps file an issue so that we document this current limitation and perhaps address it in the future, if it will become a frequent issue or when GitHub Actions gain support for triggering workflows manually and we can just document how to do that when needed.