Use explicit tags for Postgres' dockerImage

We should stop using latest tags (in this case, not really latest, but 15, 14, etc. -- this is the same as latest just named differently) because they lead to confusion such as in #538.

Instead, let's use explicit tags for DBLab Community Edition. 3.4.0 is already out so it's too late to fix it, for next releases we'll make sure we know what Postgres version we're using. This will mean:

• DBLab 3.4.1 will be released with connection to Postgres images 0.3.0
• If we update Postgres images, it's ok to release new DBLab minor version (e.g. if we release Postgres image 0.3.1, we hard-code it so DBLab 3.4.1 will be shipped with it)
• In DBLab release notes, we'll mention that it goes with Postgres images that include certain Postgres minor versions, as well as certain list of extensions, with their certain versions too
  • Example: our Postgres image 0.3.0 has pgvector v0.4.4, currently the latest – but soon, pgvector v0.5.0 will be release with very desired HNSW support https://github.com/pgvector/pgvector/blob/master/CHANGELOG.md#050-unreleased -- so we could stamp Postgres image 0.3.1 and release DBLab 3.4.1 with it)

The use of different images is still possible – users could just adjust configs for it, and with explicit tagging, it's more straightforward.