feat(solutions): add is-palindrome/c/function

templates/docker/c/Dockerfile

@@ -1,4 +1,4 @@
 FROM gcc:11.2.0
 COPY ./ ./
-RUN gcc solution.c --output=solution
+RUN gcc *.c --output=solution
 CMD ["./solution"]

templates/solution/c/solution.c

@@ -6,5 +6,5 @@ int main() {
   while (scanf("%s", &input) != EOF) {
     printf("Hello, %s!", input);
   }
-  return 0;
+  return EXIT_SUCCESS;
 }