This patch extends dcmerge to support the command attribut of a defined service.
For example:
```yaml
services:
busybox
command: [ "/usr/bin/cp", "--recursive", "--force", "/tmp/bar.txt", "/tmp/foo.txt"]
image: library/busybox:latest
```
The command attribute is interpreted as a whole. This means that individual
arguments are not merged as a comparison, as this would change the meaning of
the command attribute.
The current implementation of the method DependsOnContainer.MarshalYAML()
transform the short syntax into the long syntax. More about booth version
types of depends_on is described here:
- https://docs.docker.com/reference/compose-file/services/#short-syntax-1
- https://docs.docker.com/reference/compose-file/services/#long-syntax-1
Other applications are not compatible with the long syntax. For this reason the
MarshalYAML method has been adapted to take care of the specific syntax.
As documented of the long syntax, `depends_on.<dependency>.condition: service_started`
is the same as `depends_on: [ 'dependency' ]`, the long syntax will be shortened
when no other condition type of a dependency is specified.
