Standalone mode support

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

src/docker.ts

@@ -0,0 +1,19 @@
+import * as exec from '@actions/exec';
+
+export async function isAvailable(): Promise<boolean> {
+  return await exec
+    .getExecOutput('docker', undefined, {
+      ignoreReturnCode: true,
+      silent: true
+    })
+    .then(res => {
+      if (res.stderr.length > 0 && res.exitCode != 0) {
+        return false;
+      }
+      return res.exitCode == 0;
+    })
+    // eslint-disable-next-line @typescript-eslint/no-unused-vars
+    .catch(error => {
+      return false;
+    });
+}