circleci: Use pipeline parameters to reference docker image tags

[bshastry]

Currently, we reference docker images via their full name. Example

https://github.com/ethereum/solidity/blob/cf13339ad892f875db004577bae60115ef6e391d/.circleci/config.yml#L109

This makes it somewhat annoying when docker images are updated, and new tags need to be referenced. One must replace each and every reference manually, a practice that is prone to error.

This issue tracks support for the use of pipeline parameters in the circleci config file. One proposal would be to define such parameters for all the tags we use--- ubuntu1904-<rev>, ubuntu1804-<rev> and ubuntu1904-clang-<rev> where rev is another parameter that equals the currently used revision tag. This way, the only change required post a docker image update is to update the rev pipeline parameter. This is less prone to error and significantly reduces changes in the config file.

Note that the use of pipeline parameters requires us to update circle ci configuration version from 2 (current value) to 2.1 (see https://github.com/CircleCI-Public/api-preview-docs/blob/master/docs/pipeline-parameters.md#pipeline-parameters)

[chriseth]

Is it not possible to use a yaml feature for that?

[bshastry]

It should be possible to use yaml aliases as well as we already do. I'll open a PR for this shortly