ci: Migrate to detect-versions with namespaced cache keys

Replace local detect-runner-os action with external detect-versions@v1 to
reduce custom action maintenance. Add architecture detection for future
cross-platform support and namespace all cache keys with "continuwuity-"
prefix to prevent collisions with other projects on shared runners.

Updates cache mount IDs in Dockerfiles to match the new namespacing
convention, ensuring consistent cache isolation across CI and Docker builds.

.forgejo/actions/prepare-docker-build/action.yml

@@ -121,7 +121,7 @@ runs:
           .cargo/git/checkouts
           .cargo/registry
           .cargo/registry/src
-        key: rust-registry-image-${{hashFiles('**/Cargo.lock') }}
+        key: continuwuity-rust-registry-image-${{hashFiles('**/Cargo.lock') }}
 
     - name: Cache cargo target
       if: ${{ env.BUILDKIT_ENDPOINT == '' }}
@@ -130,7 +130,7 @@ runs:
       with:
         path: |
           cargo-target${{ env.CPU_SUFFIX }}-${{ inputs.slug }}-${{ inputs.profile }}
-        key: cargo-target${{ env.CPU_SUFFIX }}-${{ inputs.slug }}-${{ inputs.profile }}-${{hashFiles('**/Cargo.lock') }}-${{steps.rust-toolchain.outputs.rustc_version}}
+        key: continuwuity-cargo-target${{ env.CPU_SUFFIX }}-${{ inputs.slug }}-${{ inputs.profile }}-${{hashFiles('**/Cargo.lock') }}-${{steps.rust-toolchain.outputs.rustc_version}}
 
     - name: Cache apt cache
       if: ${{ env.BUILDKIT_ENDPOINT == '' }}
@@ -139,7 +139,7 @@ runs:
       with:
         path: |
           var-cache-apt-${{ inputs.slug }}
-        key: var-cache-apt-${{ inputs.slug }}
+        key: continuwuity-var-cache-apt-${{ inputs.slug }}
 
     - name: Cache apt lib
       if: ${{ env.BUILDKIT_ENDPOINT == '' }}
@@ -148,7 +148,7 @@ runs:
       with:
         path: |
           var-lib-apt-${{ inputs.slug }}
-        key: var-lib-apt-${{ inputs.slug }}
+        key: continuwuity-var-lib-apt-${{ inputs.slug }}
 
     - name: inject cache into docker
       if: ${{ env.BUILDKIT_ENDPOINT == '' }}