build: Add multithreading to make

nproc is a standard util that prints amount of available processes.
-j is a flag that specifies how many cores can be utilized by `make` (default is 1)
-j$(nproc) allows to use all the available machine cores, potentially significantly speeding up completion of CI actions

Dockerfile.debian

@@ -47,7 +47,7 @@ RUN git clone -c http.sslverify=false https://github.com/strophe/libstrophe
 WORKDIR /usr/src/libstrophe
 RUN ./bootstrap.sh
 RUN ./configure --prefix=/usr
-RUN make
+RUN make -j$(nproc)
 RUN make install
 
 WORKDIR /usr/src/profanity

Dockerfile.fedora

@@ -60,7 +60,7 @@ RUN git clone -c http.sslverify=false https://github.com/strophe/libstrophe
 WORKDIR /usr/src/libstrophe
 RUN ./bootstrap.sh
 RUN ./configure --prefix=/usr
-RUN make
+RUN make -j$(nproc)
 RUN make install
 
 RUN mkdir -p /usr/src/profanity

Dockerfile.ubuntu

@@ -49,7 +49,7 @@ RUN git clone -c http.sslverify=false https://github.com/strophe/libstrophe
 WORKDIR /usr/src/libstrophe
 RUN ./bootstrap.sh
 RUN ./configure --prefix=/usr
-RUN make
+RUN make -j$(nproc)
 RUN make install
 
 WORKDIR /usr/src/profanity