Docker: Change word in file at container startup - docker

Question

I'm creating a docker image for our fluentd.
The image contains a file called http_forward.conf
It contains:
<store>
type http
endpoint_url ENDPOINTPLACEHOLDER
http_method post # default: post
serializer json # default: form
rate_limit_msec 100 # default: 0 = no rate limiting
raise_on_error true # default: true
authentication none # default: none
username xxx # default: ''
password xxx # default: '', secret: true
</store>
So this is in our image. But we want to use the image for all our environments. Specified with environment variables.
So we create an environment variable for our environment:
ISSUE_SERVICE_URL = http://xxx.dev.xxx.xx/api/fluentdIssue
This env variable contains dev on our dev environment, uat on uat etc.
Than we want to replace our ENDPOINTPLACEHOLDER with the value of our env variable. In bash we can use:
sed -i -- 's/ENDPOINTPLACEHOLDER/'"$ISSUE_SERVICE_URL"'/g' .../http_forward.conf
But how/when do we have to execute this command if we want to use this in our docker container? (we don't want to mount this file)

Answer 1

We did that via ansible coding.
Put the file http_forward.conf as template, and deploy the change depend on the environment, then mount the folder (include the conf file) to docker container.
ISSUE_SERVICE_URL = http://xxx.{{ environment }}.xxx.xx/api/fluentdIssue
playbook will be something like this, I don't test it.
- template: src=http_forward.conf.j2 dest=/config/http_forward.conf mode=0644
- docker:
name: "fluentd"
image: "xxx/fluentd"
restart_policy: always
volumes:
- /config:/etc/fluent

Answer 2

In your DockerFile you should have a line starting with CMD somewhere. You should add it there.
Or you can do it cleaner: set the CMD line to call a script instead. For example CMD ./startup.sh. The file startup.sh will then contain your sed command followed by the command to start your fluentd (I assume that is currently the CMD).