Update pipeline docker images from docker tags to docker shasum

[fmalmeida]

This relates to changing the docker config so, it uses docker shasum instead of tags, meaning that we can ensure that the pipeline always runs with the exact version of the docker image.

Sometimes, a docker image using a tag, can be updated due to bug fixes as happened in #96 however, if already available in the user's system, nextflow will not try to download it again.

However, using docker shasum, we can ensure making the pipeline always use the exact version of the pushed image, thus, guaranteeing automatic update by nextflow if a bugfix is made, even if the same tag is already available, however, with a different shasum, it is not considered the same for the pipeline.

E.g.

form this

// container for python tools
    withLabel: 'python' {
      container = 'fmalmeida/bacannot:v3.2_pyenv'
    }

• to this*

// container for python tools
    withLabel: 'python' {
      container = 'fmalmeida/bacannot@sha256:bc217d0033d2a553a6a389e9fa3fe4912d6d11c07954bfe8d138fcdbfa2d43ef'
    }

[fmalmeida]

Merged.