fix: handle IMDSv2 auth when checking if we are running on EC2 instan…

…ce (#193)

During startup a check is performed that the necessary `env` variables are present in the container.
If using in K8s running on EC2 instances with IMDSv2 enabled/enforced the checks will fail:

```shell
/docker-entrypoint.sh: Launching /docker-entrypoint.d/00-check-for-required-env.sh
Required S3_ACCESS_KEY_ID environment variable missing
Required S3_SECRET_KEY environment variable missing
```

The reason for this are the following lines:
https://github.com/nginxinc/nginx-s3-gateway/blob/bb03e8889025b76e0af51f40882ca67672d18d28/common/docker-entrypoint.d/00-check-for-required-env.sh#L47-L48

Calling the `http://169.254.169.254` endpoint requires a token (which is correctly done here for example): https://github.com/nginxinc/nginx-s3-gateway/blob/bb03e8889025b76e0af51f40882ca67672d18d28/common/etc/nginx/include/awscredentials.js#L345-L347 
I guess we need to update `00-check-for-required-env.sh`  to fetch the token first as well, like:

```shell
elif TOKEN=`curl -X PUT --silent --fail --connect-timeout 2 --max-time 2 "http://169.254.169.254/latest/api/token" -H "X-aws-ec2-metada
ta-token-ttl-seconds: 21600"` && curl  -H "X-aws-ec2-metadata-token: $TOKEN" --output /dev/null --silent --head --fail --connect-timeout
2 --max-time 5 "http://169.254.169.254"; then 
```

Reference: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/instancedata-data-retrieval.html#instance-metadata-returns

common/docker-entrypoint.d/00-check-for-required-env.sh

@@ -44,7 +44,7 @@ elif [[ -v AWS_SESSION_TOKEN ]]; then
 # b) Using Instance Metadata Service (IMDS) credentials, if IMDS is present at http://169.254.169.254.
 #    See https://docs.aws.amazon.com/sdkref/latest/guide/feature-imds-credentials.html.
 #    Example: We are running inside an EC2 instance.
-elif curl --output /dev/null --silent --head --fail --connect-timeout 2 --max-time 5 "http://169.254.169.254"; then
+elif TOKEN=`curl -X PUT --silent --fail --connect-timeout 2 --max-time 2 "http://169.254.169.254/latest/api/token" -H "X-aws-ec2-metadata-token-ttl-seconds: 21600"` && curl  -H "X-aws-ec2-metadata-token: $TOKEN" --output /dev/null --silent --head --fail --connect-timeout 2 --max-time 5 "http://169.254.169.254"; then 
   echo "Running inside an EC2 instance, using IMDS for credentials"
 
 # c) Using assume role credentials. This is indicated by AWS_WEB_IDENTITY_TOKEN_FILE being set.