Make docker build multi-stage

Dockerfile

@@ -1,43 +1,15 @@
 # This builds a dockerfile containing a working copy of PySR
 # with all pre-requisites installed.
 
+ARG JLVERSION=1.8.2
 ARG PYVERSION=3.10.8
 
+FROM julia:$JLVERSION AS jl
 FROM python:$PYVERSION
 
-# metainformation
-LABEL org.opencontainers.image.authors = "Miles Cranmer"
-LABEL org.opencontainers.image.source = "https://github.com/MilesCranmer/PySR"
-LABEL org.opencontainers.image.licenses = "Apache License 2.0"
-
-# Need to use ARG after FROM, otherwise it won't get passed through.
-ARG JLVERSION=1.8.2
-
-ENV PYVERSION $PYVERSION
-ENV JLVERSION $JLVERSION
-
-# arm64:
-# https://julialang-s3.julialang.org/bin/linux/aarch64/1.8/julia-1.8.2-linux-aarch64.tar.gz
-# amd64:
-# https://julialang-s3.julialang.org/bin/linux/x64/1.8/julia-1.8.2-linux-x86_64.tar.gz
-
-RUN export JULIA_VER=$(echo $JLVERSION | cut -d '.' -f -2) && \
-    export ARCH=$(arch | sed 's/x86_64/amd64/' | sed 's/aarch64/arm64/') && \
-    if [ "$ARCH" = "amd64" ]; then \
-        export BASE_URL="https://julialang-s3.julialang.org/bin/linux/x64/$JULIA_VER" && \
-        export FULL_URL=$BASE_URL/julia-$JLVERSION-linux-x86_64.tar.gz; \
-    elif [ "$ARCH" = "arm64" ]; then \
-        export BASE_URL="https://julialang-s3.julialang.org/bin/linux/aarch64/$JULIA_VER"; \
-        export FULL_URL=$BASE_URL/julia-$JLVERSION-linux-aarch64.tar.gz; \
-    else \
-        echo "Download link for architecture ${ARCH} not found. Please add the corresponding Julia download URL to this Dockerfile." && \
-        exit 1; \
-    fi && \
-    wget -nv $FULL_URL -O julia.tar.gz && \
-    tar -xzf julia.tar.gz && \
-    rm julia.tar.gz && \
-    mv julia-$JLVERSION /opt/julia && \
-    ln -s /opt/julia/bin/julia /usr/local/bin/julia
+# Merge Julia image:
+COPY --from=jl /usr/local/julia /usr/local/julia
+ENV PATH="/usr/local/julia/bin:${PATH}"
 
 # Install IPython and other useful libraries:
 RUN pip install ipython matplotlib
@@ -57,4 +29,9 @@ RUN pip3 install .
 # Install Julia pre-requisites:
 RUN python3 -c 'import pysr; pysr.install()'
 
+# metainformation
+LABEL org.opencontainers.image.authors = "Miles Cranmer"
+LABEL org.opencontainers.image.source = "https://github.com/MilesCranmer/PySR"
+LABEL org.opencontainers.image.licenses = "Apache License 2.0"
+
 CMD ["ipython"]