Add new GATK image built from nightly snapshot 2026-03-01

[EddieLF]

Build a GATK image using the GATK Nightly Snapshot from 2026-03-01, separately to the one in the gatk folder, which installs the latest GATK release (4.6.2.0).

The reason for building a new GATK image from the "unstable" nightly builds is because these builds include htsjdk 4.3.0, while the latest release does not - meaning no support for CRAM 3.1 in the official release.

The Dockerfile in this PR does things mostly the same as the other gatk image, except for a few differences.

• Build from the Docker image copied into our artifact repo by this script.
• Explicitly set / unset PATH variables when building BCFtools from source - otherwise the Conda env settings from the original GATK Dockerfile get in the way of the install. Similarly, explicitly set PKG_CONFIG_PATH before the make command.
• Install Google-cloud sdk using curl not wget - this resolved some issues with piping to stdout
• General code cleanup and refactor with neater stages, layers, and comments

(A lot of credit for this goes to Gemini)

[github-actions]

🐳 Expected Tags

Some Dockerfiles have been modified in this PR. New tags are expected to be deployed.
The table below shows what the tags are expected to be on merging this PR.

Name	Tag
gatk_nightly	4.6.2.0-21-ge8c49f600-NIGHTLY-SNAPSHOT-1