fix image build for statictendril

This change fixes some minor errors in the blubber image build:

• move used directory from /srv/org/wikimedia/static-tendril to /srv/app/html/statictendril
• remove redirect to 443
• remove custom and errorlog, use /dev/stderr
• rename html file form static-tendril-index.html to index.html
• rename blubber variant from bienvenida to statictendril
• user port 8080 instead of 80

Bug: T340182