Install OpenBLAS in final image

Previously, CVXOPT was unusable on the arm64 variant with this error:

    ImportError: liblapack.so.3: cannot open shared object file: No such file or directory

This makes sense because the CVXOPT installation instructions state that
BLAS and LAPACK are required¹ (these are provided by the OpenBLAS
package). In c151560, they were only
installed in the builder image since I did not realize they were
required during run time.

¹ https://cvxopt.org/install/#building-and-installing-from-source

Dockerfile

@@ -313,6 +313,20 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 RUN curl -fsSL https://deb.nodesource.com/setup_14.x | bash - \
  && apt-get update && apt-get install -y nodejs
 
+# Used for platform-specific instructions
+ARG TARGETPLATFORM
+
+# Install CVXOPT deps on linux/arm64
+# CVXOPT, an Augur dependency, was built separately above without runtime deps¹
+# packaged like they are for the amd64 wheel.
+#
+# ¹ https://cvxopt.org/install/#building-and-installing-from-source
+RUN if [[ "$TARGETPLATFORM" == linux/arm64 ]]; then \
+      apt-get update && apt-get install -y --no-install-recommends \
+          libopenblas-base \
+      ; \
+    fi
+
 # Configure bash for interactive usage
 COPY bashrc /etc/bash.bashrc