Workaround mergo merging panic for volumes

[ulyssessouza]

This should fix docker-archive/compose-cli#1261 and docker/cli#2916

The process should probably be applied to other maps, like networks, secrets, configs and extensions

[ndeloof]

If we adopt the "pointer" approach, then we need to do the same on all map[]foo types

[ulyssessouza]

@ndeloof You are right!
This is just a "Draft" PR to check if there is something missing before I do the rest of the types

[ndeloof]

Reading darccio/mergo#90 (comment) it seems this would not be the sivler bullet you expected: non-empty source would be fully replaced by override, not merged.

[ndeloof]

I ran my own debug session investigating docker-archive/compose-cli#1261 ...
surprisingly, mapstruct is able to set a map field (see https://github.com/mitchellh/mapstructure/blob/master/mapstructure.go#L846) while mergo fails doing the same https://github.com/imdario/mergo/blob/cd55aeac72a19c6d5826061d5125d72b252a12eb/merge.go#L104
but that's all voodoo to me (ㆆ _ ㆆ)

I noticed (trying to get a minimal test set) that issue happens as compose override file defines driver_opts while the source has this map initial value set to nil
i.e adding an empty map in the yaml doc fix this issue:

volumes:
  data:
    driver_opts: {}

so a possible fix, without digging into mergo internal reflect voodoo is to enforce all maps to be set non-nil.
mapstructure's ZeroFields could help us here, but unfortunately this only applies to keys in source, and other fields are skipped (https://github.com/mitchellh/mapstructure/blob/master/mapstructure.go#L1350-L1353)
Need more investigations :P

[ndeloof]

tested with

volumes:
  data:

• override:

volumes:
  data:
    driver: azure_file
    driver_opts:
      share_name: test-share
      storage_account_name: my-storage-account

as reported in initial issue

=> panic persists

by my own investigation, root cause is DriverOpts is a nil map and mergo fails to set a value.
Can't find a satisfying fix so far.

[ulyssessouza]

@ndeloof Just waiting on merge of darccio/mergo#177 to bump mergo to fix the nil issue

[ndeloof]

@ulyssessouza darccio/mergo#177 has been merged

[ulyssessouza]

@ndeloof Yep! And I've just bumped mergo in this PR too

[ndeloof]

ack, but with the fix you should not need to use *types.VolumeConfig anymore