This commit introduces the building of the Docker images as bonafide
packaging formats alongside our existing archive and packaging
distributions. This build is migrated from a dedicated repository, and
converted to Gradle in the process.

* master: (133 commits)
  SNAPSHOT: Increase Timeout to Stabilize Test (elastic#36294)
  Fix get certificates HLRC API (elastic#36198)
  Avoid shutting down the only master (elastic#36272)
  Fix typo in comment
  Fix total hits serialization of the search response (elastic#36290)
  Fix FullClusterRestartIT#testRollupIDSchemeAfterRestart
  Mute FullClusterRestartIT#testRollupIDSchemeAfterRestart as we await a fix.
  [Docs] Add Profile API limitations (elastic#36252)
  Make sure test don't use Math.random for reproducability (elastic#36241)
  Fix compilation
  ingest: support default pipeline through an alias (elastic#36231)
  Zen2: Rename tombstones to exclusions (elastic#36226)
  [Zen2] Hide not recovered state (elastic#36224)
  Test: mute testDataNodeRestartWithBusyMasterDuringSnapshot
  Test: mute testSnapshotAndRestoreWithNested
  Revert "Test: mute failing mtermvector rest test"
  Test: mute failing mtermvector rest test
  add version 6.5.3 (elastic#36268)
  Make hits.total an object in the search response (elastic#35849)
  [DOCS] Fixes broken link in execute watch
  ...