Permit '=' separator and '[ipv6]' in 'extra_hosts'

Fixes docker/cli#4648

Update the 'extra_hosts' element to align with changes in 'docker/cli'
and 'docker/buildx' that make it easier to specify IPv6 addresses in
the '--add-host' option, by permitting 'host=ip' in addition to 'host:ip',
and allowing square brackets around the address.

For example:

  extra_hosts:
    - "somehost=162.242.195.82"
    - "myhostv6=::1"
    - "anotherhostv6=[2001:4860:4860::8888]"

Signed-off-by: Rob Murray <rob.murray@docker.com>

05-services.md

@@ -850,12 +850,23 @@ external_links:
 `extra_hosts` adds hostname mappings to the container network interface configuration (`/etc/hosts` for Linux).
 
 ### Short syntax
-Short syntax uses plain strings in a list. Values must set hostname and IP address for additional hosts in the form of `HOSTNAME:IP`.
+Short syntax uses plain strings in a list. Values must set hostname and IP address for additional hosts in the form of `HOSTNAME=IP`.
+
+```yml
+extra_hosts:
+  - "somehost=162.242.195.82"
+  - "otherhost=50.31.209.229"
+  - "myhostv6=::1"
+```
+
+IPv6 addresses can be enclosed in square brackets and, for backward
+compatibility, the separator `:` can be used. For example:
 
 ```yml
 extra_hosts:
   - "somehost:162.242.195.82"
   - "otherhost:50.31.209.229"
+  - "myhostv6:[::1]"
 ```
 
 ### Long syntax
@@ -865,6 +876,7 @@ Alternatively, `extra_hosts` can be set as a mapping between hostname(s) and IP(
 extra_hosts:
   somehost: "162.242.195.82"
   otherhost: "50.31.209.229"
+  myhostv6: "::1"
 ```
 
 Compose creates a matching entry with the IP address and hostname in the container's network
@@ -873,6 +885,7 @@ configuration, which means for Linux `/etc/hosts` get extra lines:
 ```
 162.242.195.82  somehost
 50.31.209.229   otherhost
+::1             myhostv6
 ```
 
 ## group_add

build.md

@@ -289,10 +289,19 @@ Illustrative examples of how this is used in Buildx can be found
 
 `extra_hosts` adds hostname mappings at build-time. Use the same syntax as [extra_hosts](05-services.md#extra_hosts).
 
+```yml
+extra_hosts:
+  - "somehost=162.242.195.82"
+  - "otherhost=50.31.209.229"
+  - "myhostv6=::1"
+```
+IPv6 addresses can be enclosed in square brackets, and the separator `:` can be used. For example:
+
 ```yml
 extra_hosts:
   - "somehost:162.242.195.82"
   - "otherhost:50.31.209.229"
+  - "myhostv6:[::1]"
 ```
 
 Compose creates matching entry with the IP address and hostname in the container's network

spec.md

@@ -1063,12 +1063,23 @@ external_links:
 `extra_hosts` adds hostname mappings to the container network interface configuration (`/etc/hosts` for Linux).
 
 ### Short syntax
-Short syntax uses plain strings in a list. Values must set hostname and IP address for additional hosts in the form of `HOSTNAME:IP`.
+Short syntax uses plain strings in a list. Values must set hostname and IP address for additional hosts in the form of `HOSTNAME=IP`.
+
+```yml
+extra_hosts:
+  - "somehost=162.242.195.82"
+  - "otherhost=50.31.209.229"
+  - "myhostv6=::1"
+```
+
+IPv6 addresses can be enclosed in square brackets and, for backward
+compatibility, the separator `:` can be used. For example:
 
 ```yml
 extra_hosts:
   - "somehost:162.242.195.82"
   - "otherhost:50.31.209.229"
+  - "myhostv6:[::1]"
 ```
 
 ### Long syntax
@@ -1078,6 +1089,7 @@ Alternatively, `extra_hosts` can be set as a mapping between hostname(s) and IP(
 extra_hosts:
   somehost: "162.242.195.82"
   otherhost: "50.31.209.229"
+  myhostv6: "::1"
 ```
 
 Compose creates a matching entry with the IP address and hostname in the container's network
@@ -1086,6 +1098,7 @@ configuration, which means for Linux `/etc/hosts` get extra lines:
 ```
 162.242.195.82  somehost
 50.31.209.229   otherhost
+::1             myhostv6
 ```
 
 ## group_add