update go build command for Hermeto

When we switched to Hermeto from Cachito, we started seeing issues
with how the Go backend processes vendored dependencies. We were
instructed with help from the Hermeto team to add the following:

  1. source $CACHITO_ENV_FILE before `go build` cmd
  2. add -mod=readonly to `go build` command

Also, remove the gomod-vendor flag from container.yaml as it is not
used by Hermeto

https://spaces.redhat.com/pages/viewpage.action?pageId=228017926#UpstreamSources(Cachito,Hermeto,ContainerFirst)-GomodVendoring

Author: bigjust

Dockerfile

@@ -8,7 +8,7 @@ WORKDIR $REMOTE_SOURCE_DIR/app
 
 RUN dnf install -y glibc-static
 
-RUN go build -o $REMOTE_SOURCE_DIR/app/bin/snmp_notifier -ldflags "-s -X github.com/prometheus/common/version.Version=1.2.1 -X github.com/prometheus/common/version.Revision=14ba67401c61cfc2f19ebd9ace8acdcf47b4cd49 -X github.com/prometheus/common/version.Branch=master -X github.com/prometheus/common/version.BuildUser=osbs -X github.com/prometheus/common/version.BuildDate=20211104-18:55:37 -extldflags '-static'" -a -tags netgo ${REMOTE_SOURCE_DIR}/app/snmp_notifier.go
+RUN source $CACHITO_ENV_FILE && go build -mod=readonly -o $REMOTE_SOURCE_DIR/app/bin/snmp_notifier -ldflags "-s -X github.com/prometheus/common/version.Version=1.2.1 -X github.com/prometheus/common/version.Revision=14ba67401c61cfc2f19ebd9ace8acdcf47b4cd49 -X github.com/prometheus/common/version.Branch=master -X github.com/prometheus/common/version.BuildUser=osbs -X github.com/prometheus/common/version.BuildDate=20211104-18:55:37 -extldflags '-static'" -a -tags netgo ${REMOTE_SOURCE_DIR}/app/snmp_notifier.go
 
 # Build stage 2
 

container.yaml

@@ -7,5 +7,3 @@ remote_source:
     ref: 73773b728ff6147d9590e6db86e44a668bbd93d5
     pkg_managers:
       - gomod
-    flags:
-      - gomod-vendor