build: Make platform an argument

This is prepping for the option of building with multiple platforms in a
future commit, while keeping the option to build with a single platform
locally.

.github/workflows/ci.yml

@@ -38,7 +38,7 @@ jobs:
         username: nextstrain-bot
         password: ${{ secrets.GH_TOKEN_NEXTSTRAIN_BOT_MANAGE_PACKAGES }}
 
-    - run: ./devel/build -r ghcr.io -t $TAG
+    - run: ./devel/build -p linux/amd64 -r ghcr.io -t $TAG
 
     outputs:
       tag: ${{ env.TAG }}

README.md

@@ -46,10 +46,16 @@ To build this image locally,
     ./devel/build
     ```
 
-    By default, this tags the image with `latest` and pushes to
+    By default, this builds for a single platform `linux/amd64`, tags the image with `latest`, and pushes to
     `localhost:5000`. See instructions at the top of the script for additional
     options.
 
+    If the target platform is different from the build platform, set up emulation before running `./devel/build`. This can be achieved using [`tonistiigi/binfmt`](https://github.com/tonistiigi/binfmt). For example, to set up emulation for `linux/arm64`, run:
+
+    ```
+    docker run --privileged --rm tonistiigi/binfmt --install arm64
+    ```
+
 On each subsequent change during your development iterations, you can run just
 the `./devel/build` command again.
 

devel/build

@@ -14,15 +14,17 @@
 set -euo pipefail
 
 # Set default values.
+platform=linux/amd64
 registry=localhost:5000
 tag=latest
 
 # Read command-line arguments.
-while getopts "r:t:" opt; do
+while getopts "p:r:t:" opt; do
     case "$opt" in
+        p) platform="$OPTARG";;
         r) registry="$OPTARG";;
         t) tag="$OPTARG";;
-        *) echo "Usage: $0 [-r <registry>] [-t <tag>]" 1>&2; exit 1;;
+        *) echo "Usage: $0 [-p <platform>] [-r <registry>] [-t <tag>]" 1>&2; exit 1;;
     esac
 done
 
@@ -36,8 +38,6 @@ export GIT_REVISION=$(git describe --tags --abbrev=40 --always --dirty || true)
 # Calling `docker run nextstrain/base` will still only pull down the small base
 # image rather than pulling down the larger nextstrain/base-builder image.
 
-platform=linux/amd64
-
 # `buildx create` is necessary to use a driver that supports multi-platform
 # images.
 builder=nextstrain-builder