Docker build ignores "build" directory to not interfere with it (+parallel build)

[ppalucki]

When developing and testing docker images at the same, I often end up with "mkdir build" error, when building docker image, this PR fixes that.

This PR also enables parallel builds inside docker (make -j) (reduction 3m-> 38s on my machine)

Error reproduction, run from root of the project:

# local building 
(mkdir build; cd build; cmake ..; cmake --build .)

# docker build (from root of the project)
docker build .

ends up with error:

 => [internal] load build definition from Dockerfile                                                                                                                                       0.0s
 => => transferring dockerfile: 503B                                                                                                                                                       0.0s
 => [internal] load .dockerignore                                                                                                                                                          0.0s
 => => transferring context: 2B                                                                                                                                                            0.0s
 => [internal] load metadata for docker.io/library/fedora:40@sha256:4e007f288dce23966216be81ef62ba05d139b9338f327c1d1c73b7167dd47312                                                       0.0s
 => [internal] load build context                                                                                                                                                          0.6s
 => => transferring context: 40.01MB                                                                                                                                                       0.6s
 => [builder 1/4] FROM docker.io/library/fedora:40@sha256:4e007f288dce23966216be81ef62ba05d139b9338f327c1d1c73b7167dd47312                                                                 0.0s
 => CACHED [builder 2/4] RUN dnf -y install gcc-c++ git findutils make cmake                                                                                                               0.0s
 => [builder 3/4] COPY . /tmp/pcm                                                                                                                                                          0.8s
 => ERROR [builder 4/4] RUN cd /tmp/pcm && mkdir build && cd build && cmake .. && make                                                                                                     0.3s
------
 > [builder 4/4] RUN cd /tmp/pcm && mkdir build && cd build && cmake .. && make:
0.282 mkdir: cannot create directory 'build': File exists
------
Dockerfile:5
--------------------
   3 |     RUN dnf -y install gcc-c++ git findutils make cmake
   4 |     COPY . /tmp/pcm
   5 | >>> RUN cd /tmp/pcm && mkdir build && cd build && cmake .. && make
   6 |
   7 |     FROM fedora:40@sha256:4e007f288dce23966216be81ef62ba05d139b9338f327c1d1c73b7167dd47312
--------------------
ERROR: failed to solve: process "/bin/sh -c cd /tmp/pcm && mkdir build && cd build && cmake .. && make" did not complete successfully: exit code: 1

(removing outside "build" directory or using different one, solves the issue as well)

This happens becuase "build" is transfered as context to "builder" container, which tries to create "build" inside

Maybe it was intended to reause outside "build" direcotry for caching purposes, but then we're risking incoherent, no isolated builds if host environment is different from OS used in Dockerfile.

Extra gain is that with limited context, it speeds up building a little (no need to transfer unused file to container):

=> [internal] load build context                                                                                                                                                          0.6s
 => => transferring context: 40.01MB

vs with /build inside .dockerignore

 => [internal] load build context                                                                                                                                                          0.1s
 => => transferring context: 143.33kB             

dockerignore file reference: https://docs.docker.com/build/building/context/#dockerignore-files

[rdementi]

thanks a lot!