ubuntu 20.04 dockerfile error 
I'm developing a dockerfile to run the miner into a container. In last part of the build process when I'ts running the cmake it's throws me this error.

Any advice?

Thanks in advance

```
Submodule path 'external/src/robin-hood-hashing': checked out '668936f62ec025db47a3888d5a03fb3e63b25da9'
Submodule path 'tests/googletest': checked out '955c7f837efad184ec63e771c42542d37545eaef'
Removing intermediate container 00de0860cd9b
 ---> 4244d71d773b
Step 7/11 : RUN cd p2pool
 ---> Running in 883a46f7fa99
Removing intermediate container 883a46f7fa99
 ---> b7cd514fdd9a
Step 8/11 : RUN mkdir build
 ---> Running in a92e62732ca1
Removing intermediate container a92e62732ca1
 ---> 60cae7c1178e
Step 9/11 : RUN cd build
 ---> Running in 2629c4132b49
Removing intermediate container 2629c4132b49
 ---> ce3c3970cb0d
Step 10/11 : RUN cmake ..
 ---> Running in 0dffc046192e
CMake Error: The source directory "/" does not appear to contain CMakeLists.txt.
Specify --help for usage, or press the help button on the CMake GUI.
```

This is my dockerfile
```
from ubuntu:20.04

ARG DEBIAN_FRONTEND=noninteractive
ENV TZ=Europe/Madrid
RUN apt-get update
RUN apt-get install -y git build-essential cmake libuv1-dev libzmq3-dev libsodium-dev libpgm-dev libnorm-dev libgss-dev
RUN git clone --recursive https://github.com/SChernykh/p2pool
RUN cd p2pool
RUN mkdir build
RUN cd build
RUN cmake ..
RUN make -j$(nproc)
```
![image](https://user-images.githubusercontent.com/11888892/144453373-22ab54a3-497f-4f7a-98ca-9a29fcbf41ec.png)
