Remove unused arch parameter in Dockerfile and update documentation to reflect it

Author: artemdinaburg

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ jobs:
     - uses: actions/checkout@v2
     - name: Build LLVM ${{ matrix.llvm }} on ${{ matrix.ubuntu }}
       run: |
-        docker build . -t docker.pkg.github.com/trailofbits/cxx-common/llvm${{ matrix.llvm }}-ubuntu${{ matrix.ubuntu }}-amd64:latest -f Dockerfile --build-arg UBUNTU_BASE=ubuntu:${{ matrix.ubuntu }} --build-arg arch=x86 --build-arg LLVM_VERSION=${{ matrix.llvm }}
+        docker build . -t docker.pkg.github.com/trailofbits/cxx-common/llvm${{ matrix.llvm }}-ubuntu${{ matrix.ubuntu }}-amd64:latest -f Dockerfile --build-arg UBUNTU_BASE=ubuntu:${{ matrix.ubuntu }} --build-arg LLVM_VERSION=${{ matrix.llvm }}
     - name: Push Image for LLVM ${{ matrix.llvm }} on ${{ matrix.ubuntu }}
       if: github.event_name == 'push' && github.ref == 'refs/heads/master'
       run: |

Dockerfile

@@ -1,5 +1,4 @@
 ARG LLVM_VERSION=800
-ARG arch=aarch64
 ARG BOOTSTRAP=/opt/trailofbits/bootstrap
 ARG LIBRARIES=/opt/trailofbits/libraries
 ARG UBUNTU_BASE=arm64v8/ubuntu:18.04

README.md

@@ -37,16 +37,16 @@ docker pull docker.pkg.github.com/trailofbits/cxx-common/llvm800-ubuntu18.04-amd
 
 ## Building The Docker Image Yourself
 
-The Dockerfile can be built locally. It is parameterized by LLVM version, OS release, and architecture. Some examples below:
+The Dockerfile can be built locally. It is parameterized by LLVM version and OS release. Currently same-architecture builds are expected (i.e., build amd64 images on amd64 and build aarch64 images on aarch64). Some examples below:
 
 Building cxx-common for LLVM 8 on Ubuntu 18.04 on AArch64 (ARM v8 64-bit):
 ```sh
-docker build . -t cxx-common:llvm800-ubuntu18.04-aarch64 -f Dockerfile --build-arg UBUNTU_BASE=arm64v8/ubuntu:18.04 --build-arg arch=aarch64 --build-arg LLVM_VERSION=800
+docker build . -t cxx-common:llvm800-ubuntu18.04-aarch64 -f Dockerfile --build-arg UBUNTU_BASE=arm64v8/ubuntu:18.04 --build-arg LLVM_VERSION=800
 ```
 
 Building cxx-common for LLVM 4.0.1 on Ubuntu 16.04 for AMD64 (x86-64):
 ```sh
-docker build . -t cxx-common:llvm401-ubuntu16.04-amd64 -f Dockerfile --build-arg UBUNTU_BASE=ubuntu:16.04 --build-arg arch=x86 --build-arg LLVM_VERSION=401
+docker build . -t cxx-common:llvm401-ubuntu16.04-amd64 -f Dockerfile --build-arg UBUNTU_BASE=ubuntu:16.04 --build-arg LLVM_VERSION=401
 ```
 
 # Manual (non-Docker) Builds