refactor: derive git info inside Docker instead of --build-arg

- Remove .git from .dockerignore so it's available in Docker build context
- Install git in Docker image (apt-get)
- Remove ARG/ENV GIT_COMMIT_SHORT/FULL from Dockerfile
- Remove --build-arg from Makefile docker target
- Simplify build.js to use git CLI directly (no env var indirection)

build.js already had fallback logic to shell out to git; now that .git
is present in the build context, it works directly without needing
values passed in from outside Docker.

build.js

@@ -15,25 +15,21 @@ function getBuildInfo() {
     const pkg = JSON.parse(
         fs.readFileSync(path.join(__dirname, "package.json"), "utf8"),
     );
-    let commitHash = process.env.GIT_COMMIT_SHORT || "unknown";
-    if (commitHash === "unknown") {
-        try {
-            commitHash = execSync("git rev-parse --short HEAD", {
-                encoding: "utf8",
-            }).trim();
-        } catch (_) {
-            // not a git repo or git not available
-        }
+    let commitHash = "unknown";
+    try {
+        commitHash = execSync("git rev-parse --short HEAD", {
+            encoding: "utf8",
+        }).trim();
+    } catch (_) {
+        // not a git repo or git not available
     }
-    let commitHashFull = process.env.GIT_COMMIT_FULL || "unknown";
-    if (commitHashFull === "unknown") {
-        try {
-            commitHashFull = execSync("git rev-parse HEAD", {
-                encoding: "utf8",
-            }).trim();
-        } catch (_) {
-            // not a git repo or git not available
-        }
+    let commitHashFull = "unknown";
+    try {
+        commitHashFull = execSync("git rev-parse HEAD", {
+            encoding: "utf8",
+        }).trim();
+    } catch (_) {
+        // not a git repo or git not available
     }
     return {
         version: pkg.version,