[docker] Fix Docker commands hanging before docker.service starts

#### Why I did it
Fix Docker socket activation broken since PR sonic-net#2417 (January 2019). Docker commands executed before docker.service starts hang indefinitely because -H unix:// breaks systemd socket activation.

**Fixes sonic-net#25383**

##### Work item tracking
- Microsoft ADO **(number only)**:

#### How I did it
Changed `-H unix://` to `-H fd://` in  `files/docker/docker.service.conf`. Added documentation comments explaining why this flag is required to prevent future regression.

#### How to verify it

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Verified on VS image:
- docker.socket is active and triggers docker.service
- dockerd runs with -H fd:// flag
- Socket activation works correctly

#### Which release branch to backport (provide reason below if selected)

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- [x] 202305
- [x] 202311
- [x] 202405
- [x] 202411
- [x] 202505
- [x] 202511

**Reason:** This is a bug fix that has existed since January 2019. All active release branches are affected and should receive this fix.

#### Tested branch (Please provide the tested image version)

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- [x] 20201231.100
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- [x] master : `fix_docker-socket-activation-25383.0-b01504e6c` (sonic-vs.img.gz)

#### Description for the changelog

Fix Docker socket activation hang by using -H fd:// instead of -H unix:// in docker.service.conf

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N/A

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Author: mssonicbld

files/docker/docker.service.conf

@@ -1,4 +1,14 @@
+# Docker daemon systemd service override configuration
+#
+# IMPORTANT: The -H fd:// flag is required for systemd socket activation.
+# This allows Docker clients to connect to /var/run/docker.sock before
+# docker.service starts - systemd listens on the socket and passes it to
+# dockerd when the service starts. If changed to -H unix://, dockerd creates
+# a new socket instead of inheriting the one systemd was listening on, causing
+# early Docker commands to hang indefinitely.
+# See: https://github.com/sonic-net/sonic-buildimage/issues/25383
+
 [Service]
 Environment=GODEBUG=netdns=cgo
 ExecStart=
-ExecStart=/usr/bin/dockerd -H unix:// --storage-driver=overlay2 --bip=240.127.1.1/24  --iptables=false --ipv6=true --fixed-cidr-v6=fd00::/80
+ExecStart=/usr/bin/dockerd -H fd:// --storage-driver=overlay2 --bip=240.127.1.1/24  --iptables=false --ipv6=true --fixed-cidr-v6=fd00::/80