Provide and use 32-bit x86 helpers

Description

Currently only amd64 (64-bit x86), and arm helpers are provided by default. Would be useful to have 32-bit x86 helper images as well, to be able to run things without modifications on 32-bit x86 device types, such as the Intel Edison, or Siemens IOT2000 (both of them are running a container engine when using balenaOS.

One can already build their own helper images, but the flow is not obvious to start, would be great if it would just work out of the box.

Proposal

• add the required make tasks to create gitlab-runner-helpers compiled with the 386 golang arch (probably use the i386 mark)
• set that the runners on know 32-bit architectures use this new tag by default.

Links to related issues and merge requests / references

• helper images MR: !1157 (closed)
• helper architecture reporting MR: !1158 (closed)