Apache Reverse Proxy 503 Custom Page Not Working

I am running a docker container with exposed port 8001 on my host. The host is reachable from the host with localhost:8001. I am also running an Apache server on port 8000.

My Apache configuration looks like this (edited)

<VirtualHost *:8000>
        ProxyPreserveHost On

        DocumentRoot /var/www/html
        ErrorDocument 503 /index.html
        ProxyPass / http://0.0.0.0:8001/
        ProxyPassReverse / http://0.0.0.0:8001/

        ServerName localhost
</VirtualHost>

I want to serve custom 503 error page when docker container is down, but I keep getting default error page.

Error logs from /var/log/apache2/error.log

[Sat May 23 18:07:25.126481 2020] [proxy:error] [pid 8769:tid 140193025681152] (111)Connection refused: AH00957: HTTP: attempt to connect to 0.0.0.0:8001 (0.0.0.0) failed
[Sat May 23 18:07:25.126524 2020] [proxy_http:error] [pid 8769:tid 140193025681152] [client 10.135.178.51:48456] AH01114: HTTP: failed to make connection to backend: 0.0.0.0
[Sat May 23 18:07:25.126577 2020] [proxy:error] [pid 8769:tid 140193025681152] (111)Connection refused: AH00957: HTTP: attempt to connect to 0.0.0.0:8001 (0.0.0.0) failed
[Sat May 23 18:07:25.126584 2020] [proxy_http:error] [pid 8769:tid 140193025681152] [client 10.135.178.51:48456] AH01114: HTTP: failed to make connection to backend: 0.0.0.0
[Sat May 23 18:07:33.957893 2020] [proxy:error] [pid 8769:tid 140193017288448] (111)Connection refused: AH00957: HTTP: attempt to connect to 0.0.0.0:8001 (0.0.0.0) failed
[Sat May 23 18:07:33.957942 2020] [proxy_http:error] [pid 8769:tid 140193017288448] [client 10.135.64.171:65456] AH01114: HTTP: failed to make connection to backend: 0.0.0.0
[Sat May 23 18:07:33.958006 2020] [proxy:error] [pid 8769:tid 140193017288448] (111)Connection refused: AH00957: HTTP: attempt to connect to 0.0.0.0:8001 (0.0.0.0) failed
[Sat May 23 18:07:33.958013 2020] [proxy_http:error] [pid 8769:tid 140193017288448] [client 10.135.64.171:65456] AH01114: HTTP: failed to make connection to backend: 0.0.0.0

When container is running, proxy works well.
What am I doing wrong? Thanks!