build(docker): pin container base images by digest for reproducible builds (#6797)

# Description of Changes

### What was changed

- Updated `BASE_VERSION` references in:
  - `docker/backend/Dockerfile`
  - `docker/embedded/Dockerfile`
  - `docker/embedded/Dockerfile.fat`
- Pinned `stirlingtools/stirling-pdf-base:1.0.2` to a specific SHA256
digest.
- Pinned the `eclipse-temurin:25-jre-noble` image used in the
`jar-extract` stage to a specific SHA256 digest.
- Pinned the `ghcr.io/astral-sh/uv:python3.13-bookworm-slim` image in
`engine/Dockerfile.dev` to a specific SHA256 digest.
- Removed reliance on mutable image tags alone for these build stages.

### Why the change was made

- Ensure deterministic and reproducible Docker builds.
- Prevent unexpected changes caused by upstream image tag updates.
- Improve supply chain integrity by explicitly defining the exact image
artifacts used during builds.
- Align container build practices with security and compliance
recommendations.

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [x] I have run `task check` to verify linters, typechecks, and tests
pass
- [x] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

docker/embedded/Dockerfile

@@ -1,7 +1,7 @@
 # Stirling-PDF - Full version (embedded frontend)
 # Uses pre-built base image for fast builds
 
-ARG BASE_VERSION=1.0.2
+ARG BASE_VERSION=1.0.2@sha256:c7698687f486707ddef9e0298587ca8b44c4e96185e1bdb0c3d119eb2bf9a82e
 ARG BASE_IMAGE=stirlingtools/stirling-pdf-base:${BASE_VERSION}
 
 # Stage 1: Build the Java application and frontend