fix: restore azure devops support for dockerhub images

[stefreak]

What this PR does / why we need it:
We do not need an ENTRYPOINT, and azure devops only supports images
without it.

Which issue(s) this PR fixes:

Fixes #4818

Special notes for your reviewer:

[stefreak]

@ITHedgeHog is there anything else that comes to your mind that could cause incompatibilities with Azure DevOps, or should this change suffice?
It should make all images compatible with Azure Devops now, not just the garden-azure image.

[ITHedgeHog]

That looks fine to me, we also used to have to add the following lines to our custom Garden image:

RUN apk add --no-cache --virtual .pipeline-deps readline linux-pam  \
  && apk add bash sudo shadow \
  && apk del .pipeline-deps

I've no idea if they're still required for the image to work on Azure DevOps - just providing them here in case they turn out to be necessary.

[stefreak]

Thank you, good point! According to https://github.com/microsoft/azure-pipelines-agent/blob/master/docs/design/non-glibc-containers.md these would be required too. I'm adding this with a comment that it's for azure devops.

[worldofgeese]

Do we want to add a default CMD?