Avoid hardcoded UID and GID in docker build

The [Dockerfile](https://github.com/nextstrain/docs.nextstrain.org/blob/1da467787db9a8f9b43724f3c41264fa30ff4db6/Dockerfile) uses `UID` and `GID` args to create a user inside the container. Files are then written during build on behalf of this user, to avoid permission problems.

Mistakenly, these variables were both hardcoded to `1000`, which is fine for a single-user Linux host, bot not for macOS. 

This PR passes `UID` and `GID` from the host system as build args, so that the files are written on behalf of the the current user.

Dockerfile

@@ -3,8 +3,8 @@ FROM continuumio/miniconda3:4.9.2
 ARG DEBIAN_FRONTEND=noninteractive
 ARG USER=user
 ARG GROUP=user
-ARG UID=1000
-ARG GID=1000
+ARG UID
+ARG GID
 
 ENV TERM="xterm-256color"
 ENV HOME="/home/user"

Makefile

@@ -22,7 +22,11 @@ help:
 .ONESHELL:
 docker-html:
 	set -euox
-	docker build -t nextstrain-docs-builder --network=host .
+	docker build -t nextclade-docs-builder \
+	--network=host \
+	--build-arg UID=$(shell id -u) \
+	--build-arg GID=$(shell id -g) \
+	.
 	docker run -it --rm \
 	--name=nextstrain-docs-builder-$(shell date +%s) \
 	--init \