Document docker install for offline/firewalled servers

[Ed McDonagh]

Will need to use

• https://docs.docker.com/engine/reference/commandline/save/
• https://docs.docker.com/engine/reference/commandline/load/

‌

[Ed McDonagh]

First draft of offline install. Refs #829. [skip ci] docs only

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[Ed McDonagh]

Missed link to doc. Refs #829. [skip ci] docs only

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[Ed McDonagh]

Minor edit to doc. Added ref #829 to changes. [skip ci] docs only

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[Ed McDonagh]

Merged in issue829offlinedocker (pull request #375)

Fixes #829 but will need testing later.

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