The Docker start script runs Prisma migrations. The Prisma schema reads
`NEXT_PRIVATE_DIRECT_DATABASE_URL`, and the migrations stopped with error
P1012 when the variable was not set.
If the variable is empty, the start script sets it to
`NEXT_PRIVATE_DATABASE_URL`. This agrees with the documentation and with
docker/production/compose.yml.
Tidy the docker setup and include a Github Action
for publishing docker containers to DockerHub and
Github Container Registry.
Also add a small README file with docker hosting instructions.
ephraimduncan
Lucas Smith
Ephraim Duncan