ci: probe system packages before building

A failure anywhere in the chain currently looks the same from outside, so there
is no way to tell a package mirror problem from a compile error.

- run the fontconfig install as the first step
- split restore, build and test into named steps
- point the package cache at the shared pipeline volume

.woodpecker/ci.yaml

@@ -2,28 +2,55 @@ when:
   - event: pull_request
   - event: push
 
+variables:
+  # global.json pins the .NET 10 SDK (10.0.0, rollForward latestMinor).
+  - &sdk_image mcr.microsoft.com/dotnet/sdk:10.0
+  # Steps run in separate containers, so the package cache lives on the shared
+  # pipeline volume instead of a per-container home directory.
+  - &dotnet_env
+    NUGET_PACKAGES: /woodpecker/nuget
+    DOTNET_CLI_TELEMETRY_OPTOUT: "1"
+    DOTNET_NOLOGO: "1"
+  - &k8s_resources
+    kubernetes:
+      serviceAccountName: jellyfin-ha-src
+      resources:
+        requests:
+          memory: 2Gi
+          cpu: 2
+        limits:
+          memory: 8Gi
+          cpu: 4
+
 steps:
-  # Restore, build and test the full solution. global.json pins the .NET 10
-  # SDK (10.0.0, rollForward latestMinor), so build on the 10.0 SDK image.
-  - name: build-test
-    image: mcr.microsoft.com/dotnet/sdk:10.0
-    environment:
-      DOTNET_CLI_TELEMETRY_OPTOUT: "1"
-      DOTNET_NOLOGO: "1"
+  # libSkiaSharp needs fontconfig to load and the SDK image does not ship it.
+  # Probed first so a package-mirror problem is not reported as a build failure.
+  - name: sysdeps
+    image: *sdk_image
+    commands:
+      - apt-get update
+      - apt-get install -y --no-install-recommends libfontconfig1
+    backend_options: *k8s_resources
+
+  - name: restore
+    image: *sdk_image
+    environment: *dotnet_env
     commands:
       - dotnet --info
       - dotnet restore Jellyfin.sln
+    backend_options: *k8s_resources
+
+  - name: build
+    image: *sdk_image
+    environment: *dotnet_env
+    commands:
       - dotnet build Jellyfin.sln -c Release --no-restore
-      # libSkiaSharp needs fontconfig to load, and the SDK image does not ship it.
+    backend_options: *k8s_resources
+
+  - name: test
+    image: *sdk_image
+    environment: *dotnet_env
+    commands:
       - apt-get update && apt-get install -y --no-install-recommends libfontconfig1
       - dotnet test Jellyfin.sln -c Release --no-build --verbosity minimal --filter "Category!=RequiresDocker&FullyQualifiedName!~Integration"
-    backend_options:
-      kubernetes:
-        serviceAccountName: jellyfin-ha-src
-        resources:
-          requests:
-            memory: 2Gi
-            cpu: 2
-          limits:
-            memory: 8Gi
-            cpu: 4
+    backend_options: *k8s_resources