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

@@ -261,7 +261,7 @@ RUN /builder-scripts/download-repo https://github.com/nextstrain/augur "$(/build
 # used for the same reasons described above.
 WORKDIR /nextstrain/auspice
 RUN /builder-scripts/download-repo https://github.com/nextstrain/auspice release . \
- && npm install && npm run build && npm link
+ && npm install --omit dev && npm run build && npm link
 
 # Add evofr for forecasting
 RUN pip3 install evofr