fix: docker build (#1002)

[![PR App][icn]][demo] | Fix RM-XYZ
:-------------------:|:----------:

## 🧰 Changes
- fix docker build error on mac

## 🧬 QA & Testing

- [Broken on production][prod].
- [Working in this PR app][demo].


[demo]: https://markdown-pr-PR_NUMBER.herokuapp.com
[prod]: https://SUBDOMAIN.readme.io
[icn]:
https://user-images.githubusercontent.com/886627/160426047-1bee9488-305a-4145-bb2b-09d8b757d38a.svg

Makefile

@@ -18,7 +18,7 @@ endif
 
 build:
 	@echo USE_LEGACY=$(USE_LEGACY)
-	docker build --platform linux/amd64 -t markdown $(dockerfile) --build-arg REACT_VERSION=${REACT_VERSION} .
+	docker build -t markdown $(dockerfile) --build-arg REACT_VERSION=${REACT_VERSION} .
 
 # This lets us call `make run test.browser`. Make expects cmdline args
 # to be targets. So this creates noop targets out of args. Copied from