Fix: make_credentials.sh uses passwords set in the .env file

[nanjiangshu]

This PR closes #52

How to test

• Copy the file .env.example to .env
• Modify some or all of the ENVs for *_BROKER_PASSWORD and *_DB_PASSWORD
• Start the docker setup by

docker compose -f docker-compose-demo.yml up -d 

• Check if the docker setup runs properly and if the following commands returns meaningful results and the file htsnexus_test_NA12878.bam is downloaded

token=$(curl -s -k https://localhost:8080/tokens | jq -r '.[0]')
curl -s -H "Authorization: Bearer $token" http://localhost:8443/metadata/datasets | jq .
datasetID=DATASET0001
filename="htsnexus_test_NA12878.bam"
curl -s -H "Authorization: Bearer $token" http://localhost:8443/metadata/datasets/$datasetID/files  
curl -s -H "Authorization: Bearer $token" http://localhost:8443/s3/$datasetID/$filename -o "$filename"