Add BUILDPLATFORM and TARGETPLATFORM flags to speed up builds

Signed-off-by: Michael Irwin <mikesir87@gmail.com>

Author: mikesir87

Dockerfile

@@ -1,11 +1,11 @@
 # Install the base requirements for the app.
 # This stage is to support development.
-FROM python:alpine AS base
+FROM --platform=$BUILDPLATFORM python:alpine AS base
 WORKDIR /app
 COPY requirements.txt .
 RUN pip install -r requirements.txt
 
-FROM node:18-alpine AS app-base
+FROM --platform=$BUILDPLATFORM node:18-alpine AS app-base
 WORKDIR /app
 COPY app/package.json app/yarn.lock ./
 COPY app/spec ./spec
@@ -25,16 +25,16 @@ RUN apk add zip && \
     zip -r /app.zip /app
 
 # Dev-ready container - actual files will be mounted in
-FROM base AS dev
+FROM --platform=$BUILDPLATFORM base AS dev
 CMD ["mkdocs", "serve", "-a", "0.0.0.0:8000"]
 
 # Do the actual build of the mkdocs site
-FROM base AS build
+FROM --platform=$BUILDPLATFORM base AS build
 COPY . .
 RUN mkdocs build
 
 # Extract the static content from the build
 # and use a nginx image to serve the content
-FROM nginx:alpine
+FROM --platform=$TARGETPLATFORM nginx:alpine
 COPY --from=app-zip-creator /app.zip /usr/share/nginx/html/assets/app.zip
 COPY --from=build /app/site /usr/share/nginx/html