Fix cross-comp CI pipeline (ARM & MIPS32) to build against target branch instead of main (#2509)

The ARM and the MIPS32 cross-compilation jobs in .vsts-ci.yml will always target the main branch,
ignoring the branch a given PR is based on and letting potential issues sip through.
This change fixes the ARM and MIPS32 jobs so they run against whatever branch the pipeline is running
against.

build/.vsts-ci.yml

@@ -959,7 +959,7 @@ jobs:
     displayName: 'Setup'
   - script: |
       cd samples/dockerbuilds/MIPS32
-      docker build -t mipsiotbuild:latest . --network=host
+      docker build --build-arg TARGET_BRANCH=$BUILD_SOURCEBRANCH -t mipsiotbuild:latest . --network=host
       cd ..
       docker build -t mipsiotapp:latest . --network=host --file ./MIPS32/Dockerfile_adjunct
     displayName: 'Build MIPS32'
@@ -981,7 +981,7 @@ jobs:
     displayName: 'Setup'
   - script: |
       cd samples/dockerbuilds/ARM
-      docker build -t armiotbuild:latest . --network=host
+      docker build --build-arg TARGET_BRANCH=$BUILD_SOURCEBRANCH -t armiotbuild:latest . --network=host
       cd ..
       docker build -t armiotapp:latest . --network=host --file ./ARM/Dockerfile_adjunct
     displayName: 'Build ARM'

samples/dockerbuilds/ARM/Dockerfile

@@ -1,5 +1,6 @@
 FROM ubuntu:latest
 ARG DEBIAN_FRONTEND=noninteractive
+ARG TARGET_BRANCH=main
 
 #########################################
 # start from home directory
@@ -74,6 +75,8 @@ WORKDIR ..
 # Build Azure C SDK
 RUN git clone https://github.com/azure/azure-iot-sdk-c.git
 WORKDIR azure-iot-sdk-c
+RUN git fetch origin $TARGET_BRANCH
+RUN git checkout FETCH_HEAD
 RUN git submodule update --init
 RUN mkdir cmake
 WORKDIR cmake

samples/dockerbuilds/MIPS32/Dockerfile

@@ -1,4 +1,5 @@
 FROM ubuntu:latest
+ARG TARGET_BRANCH=main
 
 #########################################
 # start from home directory
@@ -80,6 +81,8 @@ WORKDIR ..
 
 RUN git clone https://github.com/azure/azure-iot-sdk-c.git
 WORKDIR azure-iot-sdk-c
+RUN git fetch origin $TARGET_BRANCH
+RUN git checkout FETCH_HEAD
 RUN git submodule update --init
 RUN mkdir cmake
 WORKDIR cmake