HDDS-12167. Cannot use sample compose definition with SHA-based image

[adoroszlai]

What changes were proposed in this pull request?

Sample docker-compose.yaml sets OZONE_IMAGE_FLAVOR to -rocky, if unset or empty. This makes it cumbersome to use the definition with images identified by commit SHA. Straightforward way results in error:

export OZONE_IMAGE=ghcr.io/adoroszlai/ozone
export OZONE_IMAGE_VERSION=8ed8e020dc69b2c6ed53a5a31cafc2af161876e2
export OZONE_IMAGE_FLAVOR=""
docker compose up -d --scale datanode=3
...
Error response from daemon: manifest unknown

Possible workaround: splitting the SHA value between OZONE_IMAGE_VERSION and OZONE_IMAGE_FLAVOR as:

export OZONE_IMAGE_VERSION=8ed8e020dc69b2c6ed53a5a31cafc2af161876
export OZONE_IMAGE_FLAVOR=e2

Since we plan to publish images also without -rocky suffix anyway, and all existing images are available both with and without (although those without it may be outdated, except 1.4.1), I think it's safe to make flavor optional (use empty value by default).

https://issues.apache.org/jira/browse/HDDS-12167

How was this patch tested?

$ OZONE_IMAGE=ghcr.io/adoroszlai/ozone
$ OZONE_IMAGE_VERSION=8ed8e020dc69b2c6ed53a5a31cafc2af161876e2

$ docker compose up -d --scale datanode=3
[+] Running 7/8
 ⠏ Network docker-ozone_default       Created
 ✔ Container docker-ozone-scm-1       Started
 ✔ Container docker-ozone-recon-1     Started
 ✔ Container docker-ozone-s3g-1       Started
 ✔ Container docker-ozone-datanode-2  Started
 ✔ Container docker-ozone-om-1        Started
 ✔ Container docker-ozone-datanode-1  Started
 ✔ Container docker-ozone-datanode-3  Started

[smengcl]

Thanks @adoroszlai . lgtm

[adoroszlai]

Thanks @smengcl for the review.