Investigate versioning of Jenkins agent docker image

[SilkeSchomann]

The versioning seems to have changed and at the moment we are using a static version since this PR: #126

[MohamedAlmaki]

We are using the direct index to locate which agent.jar to use (https://github.com/mantidproject/dockerfiles/pull/126/commits/5bd24d6d940224b2e19771078a5f4c0e4cfc8314#diff-7eb0b9afff6d572c[…]5a234c63a1e8732881L23), so it is not possible to put for example latest tag to get the latest version. We need to put the version manually in the Dockerfile; maybe there is another way to do that, unlike our current implementation