Add Nextclade v2 conditionally on platform

Nextclade/Nextalign v2 comes with pre-built binaries for multiple
platforms. Use TARGETPLATFORM [1] to construct URLs for each platform.

[1]: https://docs.docker.com/engine/reference/builder/#automatic-platform-args-in-the-global-scope

Dockerfile

@@ -166,16 +166,24 @@ COPY --from=builder \
 COPY --from=builder /build/vcftools/built/bin/    /usr/local/bin/
 COPY --from=builder /build/vcftools/built/share/  /usr/local/share/
 
+ARG TARGETPLATFORM
+
+# use /env to persist variable across RUNs
+RUN if [ "$TARGETPLATFORM" = "linux/amd64" ]; then echo "export NEXTCLADE_PLATFORM=x86_64-unknown-linux-gnu" >> /env; fi
+RUN if [ "$TARGETPLATFORM" = "linux/arm64" ]; then echo "export NEXTCLADE_PLATFORM=aarch64-unknown-linux-gnu" >> /env; fi
+
 # Add Nextalign v2
-RUN curl -fsSL https://github.com/nextstrain/nextclade/releases/latest/download/nextalign-x86_64-unknown-linux-gnu \
+RUN . /env; curl -fsSL https://github.com/nextstrain/nextclade/releases/latest/download/nextalign-$NEXTCLADE_PLATFORM \
       --output /usr/local/bin/nextalign2 \
  && chmod a+rx /usr/local/bin/nextalign2
 
 # Add Nextclade v2
-RUN curl -fsSL https://github.com/nextstrain/nextclade/releases/latest/download/nextclade-x86_64-unknown-linux-gnu \
+RUN . /env; curl -fsSL https://github.com/nextstrain/nextclade/releases/latest/download/nextclade-$NEXTCLADE_PLATFORM \
       --output /usr/local/bin/nextclade2 \
  && chmod a+rx /usr/local/bin/nextclade2
 
+RUN rm /env
+
 # Add Nextalign v1
 RUN curl -fsSL https://github.com/nextstrain/nextclade/releases/download/1.11.0/nextalign-Linux-x86_64 \
       --output /usr/local/bin/nextalign1 \