Fixes Dockerfile based examples to run from correct directory

[evantorrie]

With go-1.16, go install no longer modifies go.mod files to correct omissions of required module imports. This means all go.mod files must provide a complete and correct set of requires module imports.

While the go.mod files inside the examples directories do include the correct requires, some of the Dockerfile examples run a go install command from the corresponding instrumentation directory (one level above the examples directory). This means go install looks at the go.mod file at the instrumentation directory level instead of the go.mod file included in the examples/ directory.

This changes all such cases to set the working directory to the example directory containing the correct go.mod before running go install.

[evantorrie]

This fixes #623.

However it also indicates we may want to include checking that the examples for each contrib instrumentation build as part of the CI process, as we do in the opentelemetry-go main repo.

I've opened an issue #768 to track this

[MrAlias]

Should this include upgrading the base image to 1.16, or have I missed something?

[evantorrie]

Should this include upgrading the base image to 1.16, or have I missed something?

It could, but I thought this could be done in a separate PR. For example, in opentelemetry-go-contrib, I believe we're still testing using only go-1.14 and go-1.15, so standardizing the base images on 1.16 should probably be done in conjunction with that.

In this case, the issue is that two examples were pulling from `golang:alpine" rather than a specific version of golang

1. otelgin
2. otelmacaron

These were the only ones failing, but I took the opportunity to fix all the examples which were incorrectly trying to run go install from a parent directory with a different go.mod file than that inside the example directory, but just happened to work due to go-1.14 and go-1.15's behavior of silently fixing go.mod files that were incorrect.

In other words, this PR just fixes the previously incorrect behavior - which means they now work with any version of go.

What do you think about creating a separate issue to go through and standardize the Dockerfile based images to all be a consistent version of golang:alpine? Or perhaps use a --build-arg version which would allow building the docker-based examples using the same matrix of supported go versions as we use in the ci-build.