dev: update demo container for integration test usage

[mastercactapus]

Description:
Makes a couple small updates to the demo container for integration test use-cases:

• Added a non-admin login
• Added the ability to skip the DB seed step when not necessary

Additional Info:
To validate with docker:

# build dependencies
make bin/goalert-linux-amd64.tgz bin/linux-amd64/resetdb

# build container
docker build -t gademo -f devtools/ci/dockerfiles/demo/Dockerfile.prebuilt --build-arg ARCH=amd64 .

# Run _with_ seed data
docker run -it --rm -p 8081:8081 gademo

# Run _without_ seed data
docker run -it --rm -p 8081:8081 -e SKIP_SEED=1 gademo

Both should allow login of admin/admin123 and user/user1234, with the latter skipping the random data generation (no services, rotations, etc...)