fix: The built UI app was placed in the wrong location.

[danielballan]

Closes #1139, improves the container image build in related ways, and fixes a recent regression which broke the "secret" feature enabling ?api_key to be moved to a cookie so that API key auth could work with the React UI.

Checklist

• Add a Changelog entry
• Add the ticket number which this PR closes to the comment section

[danielballan]

The CI check meant to catch any issues with this:

tiled/canary-validator.yml

Lines 50 to 61 in 08f5d00

	- name: http
	description: Serves a UI at /ui/browse
	probe:
	httpGet:
	path: /ui/browse
	port: 8000
	initialDelaySeconds: 15
	timeoutSeconds: 10
	failureThreshold: 3
	httpHeaders:
	- name: Authorization
	value: Apikey secret

is passing on main:

 Serves a UI at /ui/browse                          [passed]

and I cannot immediately see why.

[danielballan]

Now I understand why our builds were so slow: we were copying .git into the one of the build stages. I think this may have also affected the byte-compiling time.

[danielballan]

I believe this is now in working order, but it remains to sort out why the tests passed even when the UI was missing/broken.

[padraic-shafer]

Changes look reasonable to me. Merge when ready.

[danielballan]

Investigation into tests spun off into #1143.