docker(testing): fix arch ref for linux/arm/v7

relates to chenrui333/github-action-test@eb1c78d

- `docker buildx build --progress=plain --no-cache --platform linux/arm/v7 .`
- `docker buildx build --progress=plain --no-cache --platform linux/arm/v8  .`

```
```

Signed-off-by: Rui Chen <rui@chenrui.dev>

testing/Dockerfile

@@ -4,7 +4,7 @@ RUN apt-get update && apt-get install unzip
 
 # Install Terraform
 ENV TERRAFORM_VERSION=1.1.1
-RUN case $(uname -m) in x86_64|amd64) ARCH="amd64" ;; aarch64|arm64|armv8) ARCH="arm64" ;; esac && \
+RUN case $(uname -m) in x86_64|amd64) ARCH="amd64" ;; aarch64|arm64|armv7l) ARCH="arm64" ;; esac && \
     wget -nv -O terraform.zip https://releases.hashicorp.com/terraform/${TERRAFORM_VERSION}/terraform_${TERRAFORM_VERSION}_linux_${ARCH}.zip && \
     mkdir -p /usr/local/bin/tf/versions/${TERRAFORM_VERSION} && \
     unzip terraform.zip -d /usr/local/bin/tf/versions/${TERRAFORM_VERSION} && \
@@ -13,7 +13,7 @@ RUN case $(uname -m) in x86_64|amd64) ARCH="amd64" ;; aarch64|arm64|armv8) ARCH=
 
 # Install conftest
 ENV CONFTEST_VERSION=0.28.3
-RUN case $(uname -m) in x86_64|amd64) ARCH="amd64" ;; aarch64|arm64|armv8) ARCH="arm64" ;; esac && \
+RUN case $(uname -m) in x86_64|amd64) ARCH="x86_64" ;; aarch64|arm64|armv7l) ARCH="arm64" ;; esac && \
     curl -LOs https://github.com/open-policy-agent/conftest/releases/download/v${CONFTEST_VERSION}/conftest_${CONFTEST_VERSION}_Linux_${ARCH}.tar.gz && \
     curl -LOs https://github.com/open-policy-agent/conftest/releases/download/v${CONFTEST_VERSION}/checksums.txt && \
     sed -n "/conftest_${CONFTEST_VERSION}_Linux_${ARCH}.tar.gz/p" checksums.txt | sha256sum -c && \