pack update-stack allows a user update an existing stack

[ssisil]

WHEN I run pack update-stack my.custom.stack.id --run-image my-org/run --build-image my-org/build
THEN the stack with the ID my.custom.stack.id will update the existing images for the stack in the config.toml with the ones specified
AND the user will see the following message "<stack id> successfully updated."

Notes

• Multiple run and build images can be provided
• If stack id does not exist - return the following message "<stack id> does not exist. Please run pack update-stack with an existing stack ID."

[dgodd]

I have assumed that if either run-image or build-image is not specified then that entry is unchanged. This means it is NOT possible for a user to set the array for either to empty; this seems reasonable to me (why would you want to do that after all?)

[ssisil]

@dgodd - Agreed- seems like a weird edge case to support

[ssisil]

@dgodd

Seeing this error message -

ssisil:bin ssisil$ pack update-stack io.buildpacks.stacks.scott-xenial --build-image ssisil/build:xenial-2 --run-image ssisil/run:xenial-2
Error: Missing stack: stack with id "io.buildpacks.stacks.scott-xenial" not found in pack config.toml

Should be this

"io.buildpacks.stacks.scott-xenial" does not exist. Please run pack update-stack with an existing stack ID.