Modernise Dockerfile and cache Docker build layers in CI

[tautschnig]

Modernise the Dockerfile syntax:

• Use '=' form for ENV directives (required by newer Docker versions)
• Use uppercase AS for multi-stage build aliases

Speed up the check-docker-image CI job by caching Docker build layers
via BuildKit's GitHub Actions cache backend (type=gha). On cache hit
the expensive cmake/ninja layer is skipped entirely. Uses mode=max to
cache intermediate layers from the multi-stage build.

Co-authored-by: Kiro kiro-agent@users.noreply.github.com

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[Copilot]

Pull request overview

Enable compiler caching in Docker builds (via BuildKit cache mounts + ccache) to speed up repeated rebuilds, and modernize Dockerfile syntax.

Changes:

• Add BuildKit syntax directive and a cache mount for /root/.cache/ccache, plus install ccache in the builder stage
• Configure CMake to use ccache via CMAKE_*_COMPILER_LAUNCHER
• Update CI Docker build command to pass BUILDKIT_INLINE_CACHE=1

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
Dockerfile	Adds ccache + BuildKit cache mount and updates Dockerfile syntax for multi-stage build/env vars.
.github/workflows/pull-request-checks.yaml	Adjusts Docker build invocation to pass a BuildKit inline-cache build arg.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.00%. Comparing base (7a4df92) to head (ebf3b85).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8855      +/-   ##
===========================================
- Coverage    80.01%   80.00%   -0.01%     
===========================================
  Files         1703     1703              
  Lines       188396   188396              
  Branches        73       73              
===========================================
- Hits        150738   150735       -3     
- Misses       37658    37661       +3     

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[kroening]

Is that even used?