Update content.md

[MattMorgis]

Adds some details to the README.md on Docker Hub that may help someone when using the image for the first time.

I had followed the command to run a dev server within a container, however, in order to view the UI on my host machine and to run commands against it from another container, I needed to expose the default 8200 port.

Next, when running docker run vault status, I would get:

Error checking seal status: Get https://127.0.0.1:8200/v1/sys/seal-status: dial tcp 127.0.0.1:8200: connect: connection refused

However, running docker exec -it <container-id> vault status would succeed.

It took me ~a minute~ longer than I'd like to admit until I realized the former was being run in a second container, and I needed to pass the VAULT_ADDR environment variable, which also needed to include the local IP address of the docker container running the dev server.

From there, docker run -e VAULT_ADDR='http://<container-ip>:8200' vault <command> worked perfectly. Edit: To store a secret, I also needed to pass VAULT_TOKEN

[tianon]

cc @chrishoffman @jefferai @meirish (vault image maintainers)

[tianon]

cc @chrishoffman @jefferai @meirish (vault image maintainers)

[tianon]

(I'm happy to help fix the CI formatting if the content is approved/desired by an image maintainer.)