Omit dev deps when installing Auspice

This avoids disk-heavy deps like Puppeteer, which bundle a whole
Chromium install.  Based on some rough comparisons, I expected this to
shave about 600MB from the uncompressed image size and a local test
build bore that out.

The node_modules/ tree is infamously large and bloated.  The usual
culprits are non-source files that are commonly included in package
distributions but not needed at run time.  So there is surely more we
could shave off here, but this is a huge easy start.

Dockerfile

@@ -214,7 +214,7 @@ RUN curl -fsSL -o /final/bin/nextclade2 https://github.com/nextstrain/nextclade/
 # ⁴ https://github.com/mapbox/node-pre-gyp/blob/v1.0.10/lib/node-pre-gyp.js#L186
 WORKDIR /nextstrain/auspice
 RUN /builder-scripts/download-repo https://github.com/nextstrain/auspice release . \
- && npm install && npm link
+ && npm install --omit dev && npm link
 
 # Add NCBI Datasets command line tools for access to NCBI Datsets Virus Data Packages
 RUN curl -fsSL -o /final/bin/datasets https://ftp.ncbi.nlm.nih.gov/pub/datasets/command-line/v2/linux-${TARGETARCH}/datasets