fix(ci): build only on amd64 arch

.drone.yml

@@ -11,7 +11,7 @@ steps:
 - name: markdown lint
   commands:
   - markdownlint *.md
-  image: docker.io/tmknom/markdownlint:0.23.1
+  image: docker.io/volkerraschek/markdownlint:0.30.0
   resources:
     limits:
       cpu: 50
@@ -47,6 +47,10 @@ kind: pipeline
 type: docker
 name: archlinux
 
+platform:
+  os: linux
+  arch: amd64
+
 steps:
 - name: archlinux
   image: docker.io/volkerraschek/build-image:latest
@@ -93,6 +97,10 @@ kind: pipeline
 type: docker
 name: centos
 
+platform:
+  os: linux
+  arch: amd64
+
 steps:
 - name: centos7
   image: docker.io/volkerraschek/build-image:latest
@@ -139,6 +147,10 @@ kind: pipeline
 type: docker
 name: debian
 
+platform:
+  os: linux
+  arch: amd64
+
 steps:
 - name: 8-slim
   image: docker.io/volkerraschek/build-image:latest
@@ -218,6 +230,10 @@ kind: pipeline
 type: docker
 name: fedora
 
+platform:
+  os: linux
+  arch: amd64
+
 steps:
 - name: fedora-33
   image: docker.io/volkerraschek/build-image:latest
@@ -275,6 +291,10 @@ kind: pipeline
 type: docker
 name: ubuntu
 
+platform:
+  os: linux
+  arch: amd64
+
 steps:
 - name: ubuntu-16.04
   image: docker.io/volkerraschek/build-image:latest