tools: enable content trust for mkimage-*

A few of these already had content-trust, enable for the remaineder.

Signed-off-by: Ian Campbell <ijc@docker.com>

tools/mkimage-dynamic-vhd/build.yml

@@ -1,2 +1 @@
 image: mkimage-dynamic-vhd
-disable-content-trust: true

tools/mkimage-gcp/build.yml

@@ -1,2 +1 @@
 image: mkimage-gcp
-disable-content-trust: true

tools/mkimage-iso-bios/build.yml

@@ -1,4 +1,3 @@
 image: mkimage-iso-bios
-disable-content-trust: true
 arches:
   - amd64

tools/mkimage-iso-efi/build.yml

@@ -1,3 +1,2 @@
 image: mkimage-iso-efi
-disable-content-trust: true
 network: true

tools/mkimage-vhd/build.yml

@@ -1,2 +1 @@
 image: mkimage-vhd
-disable-content-trust: true

tools/mkimage-vmdk/build.yml

@@ -1,2 +1 @@
 image: mkimage-vmdk
-disable-content-trust: true