ci: use fork clone_url for DSTACK_SRC_URL on PRs from forks

When a PR comes from a fork, the head SHA doesn't exist in the base
repo yet, so `git clone <base> && git checkout <head-sha>` inside the
Docker builder fails with "unable to read tree". Pick the head repo's
clone_url when it's available, and fall back to the base repo URL for
push events.

Author: kvinwang

.github/workflows/docker-build-check.yml

@@ -31,7 +31,7 @@ jobs:
             build-shared=build/shared
           build-args: |
             DSTACK_REV=${{ github.event.pull_request.head.sha || github.sha }}
-            DSTACK_SRC_URL=${{ github.server_url }}/${{ github.repository }}
+            DSTACK_SRC_URL=${{ github.event.pull_request.head.repo.clone_url || format('{0}/{1}', github.server_url, github.repository) }}
 
       - name: Verify pinned packages
         run: |
@@ -47,7 +47,7 @@ jobs:
             --provenance=false \
             --build-context build-shared=build/shared \
             --build-arg "DSTACK_REV=${{ github.event.pull_request.head.sha || github.sha }}" \
-            --build-arg "DSTACK_SRC_URL=${{ github.server_url }}/${{ github.repository }}" \
+            --build-arg "DSTACK_SRC_URL=${{ github.event.pull_request.head.repo.clone_url || format('{0}/{1}', github.server_url, github.repository) }}" \
             gateway/dstack-app/builder
 
       - name: Verify builder pinned packages
@@ -75,7 +75,7 @@ jobs:
             build-shared=build/shared
           build-args: |
             DSTACK_REV=${{ github.event.pull_request.head.sha || github.sha }}
-            DSTACK_SRC_URL=${{ github.server_url }}/${{ github.repository }}
+            DSTACK_SRC_URL=${{ github.event.pull_request.head.repo.clone_url || format('{0}/{1}', github.server_url, github.repository) }}
 
       - name: Verify pinned packages (qemu stage)
         run: |
@@ -91,7 +91,7 @@ jobs:
             --provenance=false \
             --build-context build-shared=build/shared \
             --build-arg "DSTACK_REV=${{ github.event.pull_request.head.sha || github.sha }}" \
-            --build-arg "DSTACK_SRC_URL=${{ github.server_url }}/${{ github.repository }}" \
+            --build-arg "DSTACK_SRC_URL=${{ github.event.pull_request.head.repo.clone_url || format('{0}/{1}', github.server_url, github.repository) }}" \
             kms/dstack-app/builder
 
       - name: Verify builder pinned packages
@@ -126,7 +126,7 @@ jobs:
             build-shared=build/shared
           build-args: |
             DSTACK_REV=${{ github.event.pull_request.head.sha || github.sha }}
-            DSTACK_SRC_URL=${{ github.server_url }}/${{ github.repository }}
+            DSTACK_SRC_URL=${{ github.event.pull_request.head.repo.clone_url || format('{0}/{1}', github.server_url, github.repository) }}
 
       - name: Verify pinned packages (runtime)
         run: |
@@ -143,7 +143,7 @@ jobs:
             --file verifier/builder/Dockerfile \
             --build-context build-shared=build/shared \
             --build-arg "DSTACK_REV=${{ github.event.pull_request.head.sha || github.sha }}" \
-            --build-arg "DSTACK_SRC_URL=${{ github.server_url }}/${{ github.repository }}" \
+            --build-arg "DSTACK_SRC_URL=${{ github.event.pull_request.head.repo.clone_url || format('{0}/{1}', github.server_url, github.repository) }}" \
             verifier
 
       - name: Verify builder pinned packages
@@ -161,7 +161,7 @@ jobs:
             --file verifier/builder/Dockerfile \
             --build-context build-shared=build/shared \
             --build-arg "DSTACK_REV=${{ github.event.pull_request.head.sha || github.sha }}" \
-            --build-arg "DSTACK_SRC_URL=${{ github.server_url }}/${{ github.repository }}" \
+            --build-arg "DSTACK_SRC_URL=${{ github.event.pull_request.head.repo.clone_url || format('{0}/{1}', github.server_url, github.repository) }}" \
             verifier
 
       - name: Verify qemu pinned packages