debug dockerfile with build-args

Dockerfile

### Build this images on top of the `ubuntu:20.04` docker image.
### Docker searches for ubuntu:20.04 on the hub.docker
FROM ubuntu:20.04

### use ubuntu's package manager system `apt-get` to install compilers
RUN apt-get update -qq  && \
    apt-get install -yq gcc
### add anyother packages you want above

ARG WITH_DEBUG

#copy file into docker
COPY test.c .

RUN if [ -z ${WITH_DEBUG+x} ];    then \
      export CFLAGS="-O3" ;        \
    else                           \
      apt-get install -yq gdb ;    \
      export CFLAGS="-g -O0" ;     \
    fi && \
    echo ${CFLAGS} && \
    gcc ${CFLAGS} -o test test.c

CMD ./test

test.c

#include<stdio.h>

int main() {
  int x;
  char string[]="Hello world";

  for(x=0; x<3; x++) { 
    printf("%d...",x);
  }

  printf("%s\n", string);

  return 0;
}