S3 backup postgres Dockerfile specifies ENV variable of S3_PATH, but image actually uses S3_PREFIX

[danimbrogno]

I think this might be a small typo and is related to #59, In the dockerfile it sets a default for the env variable:

ENV S3_PATH 'backup',

but in backup.sh, the env variable S3_PREFIX is used instead:

cat dump.sql.gz | aws s3 cp - s3://$S3_BUCKET/$S3_PREFIX/$(date +"%Y-%m-%dT%H:%M:%SZ").sql.gz || exit 2

I think the Dockerfile needs to be updated. If this is accurate I can submit a pull request.

[philstrong]

+1