Clarify usage for Docker build processes, especially with deployment keys

[j-riebe]

The current docs mention only docker/build-push-action in conjunction with deploy keys.

This might mislead users to believe, that this only applies to said Action. But the concept applies to all workflows that somehow use docker build with deploy keys.

This PR clarifies the relevant section.

[mpdude]

@j-riebe I did some minor changes and additions, would like to get your 👍.

Also, @danseeley, would you say this resolves #152?

[mpdude]

@j-riebe should we mention here that additional flags/parameters need to be given to docker to forward the SSH_AUTH_SOCK?

[j-riebe]

I think a hint would be good (although it should be "clear" from the previous section).

Would you only mention it (and expect the user to be aware of the --ssh parameter) or provide examples?
Like:

• build-push-action (copied from section above)

      - name: Build and push
        id: docker_build
        uses: docker/build-push-action@v2
        with:
          ssh: |
            default=${{ env.SSH_AUTH_SOCK }}

• docker build --ssh default=SSH_AUTH_SOCK (docker docs)
• docker compose build --ssh default=SSH_AUTH_SOCK (docker docs)

Maybe we just append the --ssh default=${{ env.SSH_AUTH_SOCK}} to the lines 132 & 133.
That should make it pretty obvious.

[danseeley]

@j-riebe I did some minor changes and additions, would like to get your 👍.

Also, @danseeley, would you say this resolves #152?

Looking for @danseely

[j-riebe]

@mpdude 👍

[danseely]

I do see that this adds some additional helpful context around how to use this action with a raw docker build command. This definitely would have helped me get up & running a bit more quickly.

However, it does not cover the issue I found and outlined in #152. Namely, I found that in my particular situation, the ssh config wasn't being written into ~/.ssh, rather into /root/.ssh, which meant I had to alter the cp command. Which seems to be internal to the functionality of the action. You can get more details about my particular context in that issue.

Full disclosure, I'm currently migrating our CI stack from Gitlab over to Github, and don't have a ton of experience with Github Actions & Workflows, so I'm learning as I go here. Thus, I can't say if the issue I ran across is particular to my specific situation or not. I can't say if my issue should be addressed in this change, that's up to @mpdude et al.

[lightningspirit]

Very good, but I think the Docker documentation is also worth mentioning:
https://docs.docker.com/engine/reference/commandline/buildx_build/#ssh

Edit: I've lost a couple of hours just debugging multi deploy keys when I found that this change was what I needed

[mpdude]

Finally got a chance to finish this. Thanks to everyone involved!