Fix Nix invocations in the README and Dockerfiles (#260)

Author: danieldk

README.md

@@ -39,12 +39,13 @@ Then quick start a build with:
 ```bash
 cd examples/activation
 nix run .#build-and-copy \
-  --override-input kernel-builder github:huggingface/kernel-builder \
-  --max-jobs 8 \
-  -j 8 \
+  --max-jobs 2 \
+  --cores 8 \
   -L
 ```
 
+Where `--max-jobs` specifies the number of build variant that should be built concurrently and `--cores` the number of CPU cores that should be used per build variant.
+
 The compiled kernel will then be available in the local `build/` directory.
 We also provide Docker containers for CI builds. For a quick build:
 

dockerfiles/Dockerfile

@@ -115,7 +115,7 @@ function build_extension {
   nix build . \
     $override \
     --max-jobs $MAX_JOBS \
-    -j $CORES \
+    --cores $CORES \
     -L
 
   echo "Build completed. Copying results to ${output_dir}"

dockerfiles/Dockerfile.user

@@ -137,7 +137,7 @@ function build_extension {
   nix build \
     .\#bundle \
     --max-jobs $MAX_JOBS \
-    -j $CORES \
+    --cores $CORES \
     -L 2>&1 | awk '{ print strftime("[%Y-%m-%d %H:%M:%S]"), $0; fflush(); }'
 
   echo "Build completed. Copying results to /home/nixuser/kernelcode/build/"