fix: stream container build output and cap max buffer size

[eysi09]

This is a quick fix for #399 (addresses point one in this comment).

A few things:

• I opted not to append the stderr to the output string in the spawn function since the error is already printed at the top of the error message. This also conforms to how the container build output was displayed before this change.
• We now only print the last hundred lines of the output in case of an error. Should this be more?
• The MAX_BUFFER_SIZE is pretty arbitrary, any thoughts?

[thsig]

Looks good to me. ~1MB sounds like an OK default max buffer size for starters too.

Any further thoughts on this one, @edvald?

[edvald]

Nope, looks good to me.