diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 70006338..f4f77b5e 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -50,8 +50,11 @@ jobs:
       - name: Short commit SHA
         id: short
         run: |
+          REPO="https://github.com/${{github.event.pull_request.head.repo.full_name}}.git"
           SHA="${{github.event.pull_request.head.sha}}${{ github.sha }}"
           SHORT_SHA=${SHA:0:7}
+          echo "repo: $REPO"
+          echo "repo=$REPO" >> $GITHUB_OUTPUT
           echo "sha: $SHORT_SHA"
           echo "sha=$SHORT_SHA" >> $GITHUB_OUTPUT
       - name: Build Docker image
@@ -60,6 +63,7 @@ jobs:
           context: .
           push: false
           build-args: |
+            WEBRCI_REPO=${{ steps.short.outputs.repo }}
             WEBRCI_SHA=${{ steps.short.outputs.sha }}
             MAKE_LIBS_TARGET=default
   build-nix:
diff --git a/Dockerfile b/Dockerfile
index 120f158f..90b8932e 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -72,7 +72,8 @@ RUN rig add 4.4.0 --without-pak
 RUN /opt/R/current/bin/R -q -e 'install.packages("pak", lib = .Library)'
 
 # Download webR and configure for LLVM flang
-RUN git clone https://github.com/r-wasm/webr.git /opt/webr
+ARG WEBRCI_REPO="https://github.com/r-wasm/webr.git"
+RUN git clone ${WEBRCI_REPO} /opt/webr
 WORKDIR /opt/webr
 ARG WEBRCI_SHA=HEAD
 RUN git checkout ${WEBRCI_SHA}