Adds .github/docker/Dockerfile.ubuntu24 -- a multi-stage image bundling gcc-14, clang-19, Qt 6.7.0, the buildcache binary from the canonical GitLab repo, per-compiler vcpkg binary caches, and per-compiler warm ResInsight buildcaches. Published to ghcr.io/<repo>/ci-ubuntu24 by a new nightly build workflow that mirrors the RHEL8 pattern, with a matching daily cleanup workflow that applies the same GHCR retention rules.
Adds .github/workflows/ResInsightWithCacheLinuxContainer.yml -- a parallel companion to the existing Linux legs in ResInsightWithCache.yml, running both compilers in a matrix inside the container with /src as the working directory so buildcache hashes line up with the warmup baked into the image.
The existing ResInsightWithCache.yml Linux legs remain unchanged during the validation phase. copilot-setup-steps.yml will be migrated to the container in a follow-up once the image is proven.
Magne Sjaastad