fix(ci): replace broken volume mount with tar pipe in verify-compilation step

The runner is containerised so the host Docker daemon cannot see its
filesystem. Replace `-v ${{ github.workspace }}:/app` with a
`tar | docker run -i ... tar xf -` pipeline to stream the workspace
into the verification container.

.forgejo/workflows/build.yaml

@@ -46,12 +46,20 @@ jobs:
 
       - name: Verify compilation
         run: |
-          docker run --rm \
-            -v "${{ github.workspace }}:/app" \
-            -w /app \
+          set -euo pipefail
+          # Pipe the workspace into the container via stdin instead of a volume
+          # mount, because the runner is itself a container whose filesystem the
+          # host Docker daemon cannot see.
+          tar --exclude='_build' --exclude='deps' --exclude='.git' \
+              --exclude='priv/static/assets' --exclude='assets/node_modules' \
+              --exclude='rust' --exclude='target' \
+              -cf - . | \
+          docker run --rm -i \
             -e MIX_ENV=prod \
+            -w /app \
             docker.io/hexpm/elixir:1.20.1-erlang-29.0.2-debian-trixie-20260518-slim \
-            sh -c '\
+            sh -euc '\
+              mkdir -p /app && cd /app && tar xf - && \
               mix local.hex --force && \
               mix local.rebar --force && \
               mix deps.get --only prod && \