Charlesthebird/read only filesystem fix

[Charlesthebird]

Steps to Reproduce this Issue

1. Set up an example environment by cloning the solo-projects repo, and running sh hack/gateway-portal/portal-server-init.sh
2. In the portal-frontend deployment, set spec.securityContext.readOnlyRootFilesystem: true to see the yarn issues that arise during the UI build.

Solution

For the dev-portal-starter, we do the UI build when the image is loaded, in order to get the latest environment variables from the deployment. This doesn't work in read-only environments. In our other UI's, like the GME UI, we do the UI build beforehand, and then make a request to the API for environment variables.

This PR adds a solution to this problem by adding a lightweight server to the UI image, which serves the UI and adds the environment variables with the EJS view engine. This makes it so that the portal server doesn't have to change to accommodate UI configuration, and the UI can stay mostly decoupled from it. This is important because:

• The UI currently supports GG + GMG backends on different URL's, and the portal server endpoint is configurable.
• This should result in the exact same experience for current users, but they would no longer have to wait for the UI to build, since it could be built when the image is built. Actually the UI image had to already have used a server when it previously did the Vite build and preview steps, so this just makes us have more control over what is being served and how.

Screenshots

Reproducing the Issue

Adding in readOnlyFilesystem: true reproduces the issue.

Adding in the Fix

Keeping readOnlyFilesystem: true and updating the image to the version that is in this PR (gcr.io/solo-public/docs/portal-frontend:read-only-filesystem-test2) fixes the issue.

BOT NOTES:
resolves https://github.com/solo-io/gloo-mesh-enterprise/issues/18336

[solo-changelog-bot]

Issues linked to changelog:
https://github.com/solo-io/gloo-mesh-enterprise/issues/18336