Build conda container without needing to run `conda activate <env>`

This PR avoids the need to explicitly activate a conda environment after creating a conda-based container with GstLAL installed. The actual CI and conda-based Docker images are simpler as a result.

One side-effect from this is that the Conda dependencies Dockerfile has been factored out into its own Dockerfile rather than embedded within the CI. I personally think this is the better move but was done previously for simplicity. The other thing to note here is that we've stopped relying on the igwn/base:conda image and instead define the base image explicitly and adapted from https://github.com/conda-forge/miniforge-images/blob/master/ubuntu/Dockerfile. A couple of reasons for this:

• The base container was changed recently and caused CI failures that were not easily resolved.
• Some of the changed introduced included adding tini as an entrypoint which appears not to be play nice with Singularity with obscure issues.