Update Docker nginx Configuration for HSTS

[mwarman]

Description

Update the nginx configuration file used by the Docker image.

The nginx server should return HTTP Strict Transport Security (HSTS) headers.

The nginx server configuration should support IPv6 addresses.

[mwarman]

Updated Dockerfile FROM to use nginx:1.15-alpine.

[mwarman]

Updated spa.conf to add HSTS header and listen for IPv6 addresses.