chore: update dockerfiles for rc6

[cylewitruk]

Description

Updates our example Dockerfiles after rc6 release.

Changes

• Bumps signer and blocklist-client images to rc6
• Adds image digests to all images in our examples
• Bumps all of our Dockerfile postgres to postgres:16.6-alpine3.21
• Bumps other dependencies in our example Dockerfiles to latest versions

Testing Information

• All tests pass and I tested pulling all of the Dockerfiles to ensure everything seems OK.
• I ran our integration tests against the updated versions as well and everything passed.

Checklist:

• I have performed a self-review of my code
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules