Zend certified PHP/Magento developer

Magento 2.3 Varnish 503 Backend fetch failed

varnish server gives back error message:

Error 503 Backend fetch failed
Backend fetch failed

Guru Meditation:
XID: 131307

varnish log info:

Varnish cache server   
# varnishlog -q 'RespStatus == 503'
*   < < Request  >> 131307
-   Begin          req 196933 rxreq
-   Timestamp      Start: 1577129611.804607 0.000000 0.000000
-   Timestamp      Req: 1577129611.804607 0.000000 0.000000
-   ReqStart       172.19.0.1 57376
-   ReqMethod      GET
-   ReqURL /index.php/admin/customer/index/index/key/a1cb5649e0727c563c92b6873c654c996b94a4729dfdb8247ef3b57350c20ff2/
-   ReqProtocol    HTTP/1.1
-   ReqHeader      Host: m2.docker
-   ReqHeader      Connection: keep-alive
-   ReqHeader      Upgrade-Insecure-Requests: 1
-   ReqHeader      User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.88 Safari/537.36
-   ReqHeader      Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
-   ReqHeader      Referer: http://m2.docker/index.php/admin/admin/system_store/index/key/3685fd86510c4b9c1f7796f25f1b7fda21adcab527d13b0c3822bcb2d207ae40/
-   ReqHeader      Accept-Encoding: gzip, deflate
-   ReqHeader      Accept-Language: en-US,en;q=0.9,zh-CN;q=0.8,zh;q=0.7
-   ReqHeader      Cookie: admin=3c2d07d3c163dd84faed04865f7e1401; form_key=CgVTpyUNua3s7Ynq; private_content_version=e105233d30e3e7553aa81958343c8dfb; AMCVS_8F99160E571FC0427F000101%40AdobeOrg=1; s_cc=true; mage-translation-storage=%7B%7D; mage-translation-file-version=%7
-   ReqHeader      X-Forwarded-For: 172.19.0.1
-   VCL_call       RECV
-   ReqURL         /index.php/admin/customer/index/index/key/a1cb5649e0727c563c92b6873c654c996b94a4729dfdb8247ef3b57350c20ff2/
-   ReqUnset       Accept-Encoding: gzip, deflate
-   ReqHeader      Accept-Encoding: gzip
-   ReqURL         /index.php/admin/customer/index/index/key/a1cb5649e0727c563c92b6873c654c996b94a4729dfdb8247ef3b57350c20ff2/
-   ReqURL         /index.php/admin/customer/index/index/key/a1cb5649e0727c563c92b6873c654c996b94a4729dfdb8247ef3b57350c20ff2/
-   ReqURL         /index.php/admin/customer/index/index/key/a1cb5649e0727c563c92b6873c654c996b94a4729dfdb8247ef3b57350c20ff2/
-   VCL_return     hash
-   VCL_call       HASH
-   VCL_return     lookup
-   Debug          "XXXX MISS"
-   VCL_call       MISS
-   VCL_return     fetch
-   Link           bereq 196935 fetch
-   Timestamp      Fetch: 1577129676.180708 64.376101 64.376101
-   RespProtocol   HTTP/1.1
-   RespStatus     503
-   RespReason     Backend fetch failed
-   RespHeader     Date: Mon, 23 Dec 2019 19:34:36 GMT
-   RespHeader     Server: Varnish
-   RespHeader     Content-Type: text/html; charset=utf-8
-   RespHeader     Retry-After: 5
-   RespHeader     X-Varnish: 196934
-   RespHeader     Age: 0
-   RespHeader     Via: 1.1 varnish-v4
-   VCL_call       DELIVER
-   RespUnset      Age: 0
-   RespUnset      Server: Varnish
-   RespUnset      X-Varnish: 196934
-   RespUnset      Via: 1.1 varnish-v4
-   VCL_return     deliver
-   Timestamp      Process: 1577129676.180742 64.376135 0.000034
-   RespHeader     Content-Length: 283
-   Debug          "RES_MODE 2"
-   RespHeader     Connection: keep-alive
-   Timestamp      Resp: 1577129676.180834 64.376226 0.000092
-   Debug          "XXX REF 1"
-   ReqAcct        1658 0 1658 175 283 458
-   End

I have used the ‘default.vcl’ from Magento 2 backend export.