Unable to delete /srv/index.php on PHP variant

```
elimbp @ Johans-MacBook-Pro ~/docker-test/caddy
└─ $ ▶ cat Dockerfile
FROM abiosoft/caddy:0.11.3-php-no-stats
RUN rm -rf /srv/index.php
RUN ls -la /srv
elimbp @ Johans-MacBook-Pro ~/docker-test/caddy
└─ $ ▶ docker image build --no-cache -t testcaddy .
Sending build context to Docker daemon 2.048kB
Step 1/3 : FROM abiosoft/caddy:0.11.3-php-no-stats
---> 31b711527125
Step 2/3 : RUN rm -rf /srv/index.php
---> Running in a7c61ae1892b
Removing intermediate container a7c61ae1892b
---> 6159ef73786a
Step 3/3 : RUN ls -la /srv
---> Running in 8c4f434f88a2
total 12
drwxr-xr-x 2 root root 4096 Feb 14 14:24 .
drwxr-xr-x 1 root root 4096 Feb 14 14:24 ..
-rw-r--r-- 1 root root 555 Feb 6 13:38 index.php <----- SHOULD BE DELETED
Removing intermediate container 8c4f434f88a2
---> 81189f83b665
Successfully built 81189f83b665
Successfully tagged testcaddy:latest
```

I am on macOS if that is to any help.