Initial commit: OCI image mounting via containers-storage

slocker-lite validates an OCI Image Layout tar, imports its layers into
containers-storage's layer store in order, and mounts the assembled image
using fuse-overlayfs, printing the resulting merged path.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/process.cpp

@@ -0,0 +1,78 @@
+// Copyright (C) 2026 Viorel Munteanu
+//
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 2 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this program; if not, write to the Free Software Foundation, Inc.,
+// 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+#include "process.h"
+
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include <cstring>
+
+#include <fmt/ranges.h>
+#include <spdlog/spdlog.h>
+
+ProcessResult run_process(const std::vector<std::string>& argv) {
+    spdlog::debug("running external command: {}", fmt::join(argv, " "));
+
+    int stdout_pipe[2];
+    if (pipe(stdout_pipe) != 0) {
+        return {-1, ""};
+    }
+
+    pid_t pid = fork();
+    if (pid < 0) {
+        close(stdout_pipe[0]);
+        close(stdout_pipe[1]);
+        return {-1, ""};
+    }
+
+    if (pid == 0) {
+        close(stdout_pipe[0]);
+        dup2(stdout_pipe[1], STDOUT_FILENO);
+        close(stdout_pipe[1]);
+
+        std::vector<char*> c_argv;
+        c_argv.reserve(argv.size() + 1);
+        for (const auto& arg : argv) {
+            c_argv.push_back(const_cast<char*>(arg.c_str()));
+        }
+        c_argv.push_back(nullptr);
+
+        execvp(c_argv[0], c_argv.data());
+        const char* msg = "run_process: execvp failed\n";
+        write(STDERR_FILENO, msg, std::strlen(msg));
+        _exit(127);
+    }
+
+    close(stdout_pipe[1]);
+
+    std::string output;
+    char buffer[4096];
+    ssize_t n;
+    while ((n = read(stdout_pipe[0], buffer, sizeof(buffer))) > 0) {
+        output.append(buffer, static_cast<size_t>(n));
+    }
+    close(stdout_pipe[0]);
+
+    int status = 0;
+    waitpid(pid, &status, 0);
+
+    int exit_code = WIFEXITED(status) ? WEXITSTATUS(status) : -1;
+    if (exit_code != 0) {
+        spdlog::warn("external command failed (exit code {}): {}", exit_code, fmt::join(argv, " "));
+    }
+    return {exit_code, output};
+}