chore: update Dockerfile with PostHog environment variables (#3154)

# Description

Please include a summary of the changes and the related issue. Please
also include relevant motivation and context.

## Checklist before requesting a review

Please delete options that are not relevant.

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented hard-to-understand areas
- [ ] I have ideally added tests that prove my fix is effective or that
my feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged

## Screenshots (if appropriate):

frontend/Dockerfile

@@ -32,6 +32,8 @@ ARG NEXT_PUBLIC_FRONTEND_URL
 ARG NEXT_PUBLIC_AUTH_MODES  # Make sure this is declared here if it influences the build
 ARG NEXT_PUBLIC_SHOW_TOKENS
 ARG NEXT_PUBLIC_PROJECT_NAME
+ARG NEXT_PUBLIC_POSTHOG_KEY
+ARG NEXT_PUBLIC_POSTHOG_HOST
 
 ENV NEXT_PUBLIC_ENV=$NEXT_PUBLIC_ENV
 ENV NEXT_PUBLIC_BACKEND_URL=$NEXT_PUBLIC_BACKEND_URL
@@ -42,6 +44,8 @@ ENV NEXT_PUBLIC_FRONTEND_URL=$NEXT_PUBLIC_FRONTEND_URL
 ENV NEXT_PUBLIC_AUTH_MODES=$NEXT_PUBLIC_AUTH_MODES 
 ENV NEXT_PUBLIC_SHOW_TOKENS=$NEXT_PUBLIC_SHOW_TOKENS
 ENV NEXT_PUBLIC_PROJECT_NAME=$NEXT_PUBLIC_PROJECT_NAME
+ENV NEXT_PUBLIC_POSTHOG_KEY=$NEXT_PUBLIC_POSTHOG_KEY
+ENV NEXT_PUBLIC_POSTHOG_HOST=$NEXT_PUBLIC_POSTHOG_HOST
 
 
 # Next.js collects completely anonymous telemetry data about general usage.
@@ -90,6 +94,10 @@ ARG NEXT_PUBLIC_SHOW_TOKENS
 ENV NEXT_PUBLIC_SHOW_TOKENS=$NEXT_PUBLIC_SHOW_TOKENS
 ARG NEXT_PUBLIC_PROJECT_NAME
 ENV NEXT_PUBLIC_PROJECT_NAME=$NEXT_PUBLIC_PROJECT_NAME
+ARG NEXT_PUBLIC_POSTHOG_KEY
+ENV NEXT_PUBLIC_POSTHOG_KEY=$NEXT_PUBLIC_POSTHOG_KEY
+ARG NEXT_PUBLIC_POSTHOG_HOST
+ENV NEXT_PUBLIC_POSTHOG_HOST=$NEXT_PUBLIC_POSTHOG_HOST
 
 # server.js is created by next build from the standalone output
 # https://nextjs.org/docs/pages/api-reference/next-config-js/output