Allow customizing #jobs for make

Dockerfile

@@ -5,6 +5,7 @@ ARG BUILD_CONTRIB_MODULES=
 FROM alpine:latest AS builder
 ARG ATHEME_VERSION
 ARG BUILD_CONTRIB_MODULES
+ARG MAKE_NUM_JOBS
 RUN mkdir /atheme-src
 
 # Install build-deps and runtime deps
@@ -26,7 +27,7 @@ RUN cd /atheme-src/libmowgli-2 && \
 # Configure and build
 RUN cd /atheme-src && \
     ./configure --prefix=/atheme $(test -z "$BUILD_CONTRIB_MODULES" || echo --enable-contrib) && \
-    make -j$(nproc) && make install
+    make -j${MAKE_NUM_JOBS:-$(nproc)} && make install
 
 
 FROM alpine:latest

README.md

@@ -30,3 +30,4 @@ For those who want to create customized builds, here are options exposed by the
 - `ATHEME_UID`: sets the UID for the Atheme user to run as. Defaults to 10000.
 - `ATHEME_VERSION`: Atheme version to pull from Git
 - `BUILD_CONTRIB_MODULES`: if set to a non-empty value, enables building contrib modules.
+- `MAKE_NUM_JOBS`: sets the `-j` flag for make. Defaults to `$(nproc)` if left empty.