Add custom docker image guide

[jbusecke]

Made this in response to @dhruvbalwada s request on slack. I tried to link to the excellent 2i2c docs as much as possible, but also explained some of the LEAP specific techniques (mostly the secret sharing).

• Make instructions to change the IMAGE values in the gh workflow more clear (see Change publish repo test-image-balwada#3 for details).
• Add more instructions on the testing That could also need some upstream clarification.

[jbusecke]

Huh, why is this not generating a preview? Ah here we go.

[github-actions]

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[jbusecke]

@dhruvbalwada if you could follow the guide above (once generated) and provide feedback, that would help a ton!