A simple way of doing it is to use
justification=raggedright together. Below is an example that creates a bolded caption with left alignment for subfigures only:
- singlelinecheck=off means that the justification setting is used
even when the caption is only a single line long.
- if singlelinecheck=on, then caption is always centered when the caption
is only one line.
- caption label is bold, the caption text normal.
- justification is raggedright (i.e. left aligned)
The above will place the caption on top of the figure left aligned. It will not place it on the side as Gonzalo example did, but I think this is what the original question was about