tagging docker images with release tag instead of commit hash

[armin-x86]

For polkadot-staking-miner, docker images are pushed with this naming convention: branchName-7 characters of the commit SHA
Exp:main-b250ced

As a result of this line in the Job.

If we change the follwing, I think it will fix the issue.

echo IMAGE_TAG=$(if [ -n "$GITHUB_REF_NAME" ]; then echo "$GITHUB_REF_NAME"; else echo "main-${GITHUB_SHA::7}"; fi) >> $GITHUB_ENV

[armin-x86]

PR