Local devcontainers (take 2)

[pinin4fjords]

Re-pitching #539 in light of recent changes. I'll pull updates to docs etc from there once people are happy with the approach.

Local devcontainers usage means making the host file paths valid within the container to appease docker-outside-docker. This is really just a case of:

1. Mount the host systems
2. Use symlinking to make /workspaces/training valid and consistent with materials, and add some options to move new terminal starting dirs to the symlink.

I'm not sure if this will all work without a local build, so

I also had to pin the java version ('latest' didn't work for me).

<     "workspaceFolder": "/workspaces/training",
---
>     "workspaceFolder": "${localWorkspaceFolder}",
>     "workspaceMount": "source=${localWorkspaceFolder},target=${localWorkspaceFolder},type=bind",
27c28
<         "NXF_HOME": "/workspaces/.nextflow",
---
>         "NXF_HOME": "/workspaces/training/.nextflow",
34c35,36
<     "onCreateCommand": "bash .devcontainer/setup.sh",
---
>     "onCreateCommand": "bash ${localWorkspaceFolder}/.devcontainer/setup.sh",
>     "postCreateCommand": "mkdir -p /workspaces && rm -rf /workspaces/training && ln -s ${localWorkspaceFolder} /workspaces/training",
40c42,44
<                 "python.defaultInterpreterPath": "/opt/conda/bin/python"
---
>                 "python.defaultInterpreterPath": "/opt/conda/bin/python",
>                 "terminal.integrated.cwd": "/workspaces/training",
>                 "terminal.integrated.defaultLocation": "workspace"

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[vdauwera]

@pinin4fjords where do we stand on this?

[pinin4fjords]

@vdauwera ready to roll from my point of view. @ewels wanted to review when he could.

[maxulysse]

Minor comment, but I followed the guide, and managed to run a different nextflow version in my local devcontainers compared to my own local install.
This is some serious magic

[pinin4fjords]

Applying maxime's little suggestions dismissed his approval, but this is tested and ready to go.