ci: map the build folder in container builds

Now that the containers all set the user id of the libgit2 build user to
the user id of the host runner, we can write to a shared build output
volume.

.github/workflows/main.yml

@@ -237,10 +237,12 @@ jobs:
         export GITTEST_NEGOTIATE_PASSWORD="${{ secrets.GITTEST_NEGOTIATE_PASSWORD }}"
 
         if [ -n "${{ matrix.platform.container.name }}" ]; then
+          mkdir build
           docker run \
               --rm \
               --user libgit2:libgit2 \
               -v "$(pwd)/source:/home/libgit2/source" \
+              -v "$(pwd)/build:/home/libgit2/build" \
               -w /home/libgit2 \
               -e ASAN_SYMBOLIZER_PATH \
               -e CC \
@@ -255,9 +257,10 @@ jobs:
               -e TSAN_OPTIONS \
               -e UBSAN_OPTIONS \
               ${{ env.docker-registry-container-sha }} \
-              /bin/bash -c "mkdir build && cd build && ../source/ci/build.sh && ../source/ci/test.sh"
+              /bin/bash -c "cd build && ../source/ci/build.sh && ../source/ci/test.sh"
         else
-          mkdir build && cd build
+          mkdir build
+          cd build
           ../source/ci/build.sh
           ../source/ci/test.sh
         fi