Update RELEASING.md (#4295)

Update the docs to reflect our current release process.

Author: kiview

RELEASING.md

@@ -1,19 +1,23 @@
 # Release process
 
-Testcontainers' release process is automated as a Travis deployment. This describes the basic steps for a project member to perform a release.
+Testcontainers' release process is semi-automated through GitHub Actions. This describes the basic steps for a project member to perform a release.
 
 ## Steps
 
 1. Ensure that the master branch is building and that tests are passing.
-1. Ensure that the [`CHANGELOG`](CHANGELOG.md) file is up to date and includes all merged features.
 1. Create a new release on GitHub. **The tag name is used as the version**, so please keep the tag name plain (e.g. 1.2.3).
-1. Check that the Travis build passed.
-1. Release of published artifacts to Bintray is fully automated.
-1. After successful publication to Bintray, the artifacts must be synced to Maven Central.
+1. The release triggers a GitHub Action workflow, but it gets mostly build using results from the Gradle remote-cache. Therefore, this should be fairly fast.
+1. Login to Sonatype to check the staging repository.
+1. Get the staging url after GitHub Action workflow finished.
+1. Manually test the release with the staging url as maven repository url (e.g. critical issues and features).
+1. Run [TinSalver](https://github.com/bsideup/tinsalver) from GitHub using `npx` to sign artifact (see [TinsSalver README](https://github.com/bsideup/tinsalver/blob/main/README.md)).
+1. Close the release in Sonatype. This will evaluate the release based on given Sontaype rules and afterwards automatically sync to Maven Central.
+1. Handcraft and polish some of the release notes (e.g. substitute combinded dependency PRs and highlight certain features).
 1. When available through Maven Central, poke [Richard North](https://github.com/rnorth) to announce the release on Twitter!
 
 ## Internal details
 
-* The process is done with Gradle and Bintray.
-* Bintray will automatically promote the release to Maven Central.
-* Travis secrets hold Bintray username/passwords that are used for publishing.
+* The process is done with GitHub Actions, TinSalver and Sonatype.
+* Sonatype will automatically promote the staging release to Maven Central.
+* Keybase needs to be installed on the developer machine.
+* GPG key of signing developer needs to be uplodaed to the Ubuntu keyserver (or other server supported by Sonatype).