fix(social): don't crash when no font is available; ship a font in the image

The social-cards plugin failed the entire build inside the container with
"SixLabors.Fonts.FontException: Cannot use the default value type instance
to create a font". Two causes:

1. ResolveFontFamily()'s last-resort `SystemFonts.Families.FirstOrDefault()`
   returns a *default* FontFamily struct (not null) when no fonts are
   installed, so the `family is null` guard missed it and CreateFont threw.
   Now we detect the default and return null, so card generation is skipped
   with a warning instead of failing the build.

2. The minimal runtime-deps container has no fonts at all. Install
   fonts-dejavu-core + fontconfig in the image so social cards actually
   render (ResolveFontFamily already prefers "DejaVu Sans").

Also enable the social plugin on docs-site so the container path is
exercised by the action self-test. Verified locally: docs-site build emits
34 social cards. Removes the temporary deploy-test workflow.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Netdocs

.github/workflows/_tmp-deploy-test.yml

@@ -12,6 +12,12 @@ RUN dotnet publish src/Netdocs.Cli -c Release -r linux-x64 --self-contained \
 
 FROM mcr.microsoft.com/dotnet/runtime-deps:11.0-preview
 WORKDIR /site
+# Ship a font so the social-cards plugin can render text. The minimal runtime-deps
+# image has no fonts, which otherwise makes SixLabors.Fonts fail with
+# "Cannot use the default value type instance to create a font".
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends fonts-dejavu-core fontconfig \
+    && rm -rf /var/lib/apt/lists/*
 COPY --from=build /app /opt/netdocs
 RUN ln -s /opt/netdocs/netdocs /usr/local/bin/netdocs
 EXPOSE 8000

Dockerfile

@@ -112,7 +112,8 @@
       { "name": "search", "options": { "lang": "en" } },
       { "name": "tags", "options": { "export": true } },
       { "name": "meta" },
-      { "name": "arithmatex" }
+      { "name": "arithmatex" },
+      { "name": "social" }
     ],
 
     "markdownExtensions": [

docs-site/appsettings.json

@@ -119,12 +119,25 @@ private Image<Rgba32> RenderCard(string title, string siteName, string descripti
     private static string Truncate(string s, int max) =>
         s.Length <= max ? s : s[..max].TrimEnd() + "…";
 
-    private static FontFamily? ResolveFontFamily()
+    private FontFamily? ResolveFontFamily()
     {
-        foreach (var name in new[] { "Open Sans", "Segoe UI", "Arial", "Helvetica", "DejaVu Sans", "Liberation Sans", "Noto Sans" })
+        foreach (var name in new[] { "Open Sans", "Roboto", "Segoe UI", "Arial", "Helvetica", "DejaVu Sans", "Liberation Sans", "Noto Sans" })
             if (SystemFonts.TryGet(name, out var family))
                 return family;
-        return SystemFonts.Families.FirstOrDefault();
+
+        // SystemFonts.Families.FirstOrDefault() returns a *default* FontFamily struct
+        // (not null) when no fonts are installed -- e.g. inside a minimal container.
+        // Calling CreateFont on that default throws "Cannot use the default value type
+        // instance to create a font" and fails the whole build. Only return a family
+        // when one genuinely exists, so callers can skip card generation instead.
+        var first = SystemFonts.Families.FirstOrDefault();
+        if (first != default)
+        {
+            _log.LogInformation("social: using fallback system font '{Font}'", first.Name);
+            return first;
+        }
+
+        return null;
     }
 
     private static Color PrimaryColor(string? name) => (name ?? "grey").ToLowerInvariant() switch