Use golang:1.15 in ECR credential helper example

[jerbob]

The example Dockerfile provided for ecr-credential-helper no longer builds on golang:1.16.

This PR simply pins the version specified in the Dockerfile to golang:1.15:

$ cat Dockerfile
FROM golang:latest

ENV CGO_ENABLED 0
ENV REPO github.com/awslabs/amazon-ecr-credential-helper/ecr-login/cli/docker-credential-ecr-login

RUN go get -u $REPO

RUN rm /go/bin/docker-credential-ecr-login

RUN go build \
 -o /go/bin/docker-credential-ecr-login \
 /go/src/$REPO

WORKDIR /go/bin/
ubuntu@ip-10-0-3-95:~/credential-helper$ docker build -t aws-ecr-dock-cred-helper .
Sending build context to Docker daemon  2.048kB
Step 1/7 : FROM golang:latest
 ---> 96129f3766cf
Step 2/7 : ENV CGO_ENABLED 0
 ---> Using cache
 ---> 905ba7abe5da
Step 3/7 : ENV REPO github.com/awslabs/amazon-ecr-credential-helper/ecr-login/cli/docker-credential-ecr-login
 ---> Using cache
 ---> 7812b2c262c2
Step 4/7 : RUN go get -u $REPO
 ---> Using cache
 ---> d598e79b0d71
Step 5/7 : RUN rm /go/bin/docker-credential-ecr-login
 ---> Using cache
 ---> 0cbf53304a18
Step 6/7 : RUN go build  -o /go/bin/docker-credential-ecr-login  /go/src/$REPO
 ---> Running in 2dad4197b560
go: go.mod file not found in current directory or any parent directory; see 'go help modules'
The command '/bin/sh -c go build  -o /go/bin/docker-credential-ecr-login  /go/src/$REPO' returned a non-zero
code: 1

$ cat Dockerfile
FROM golang:1.15

ENV CGO_ENABLED 0
ENV REPO github.com/awslabs/amazon-ecr-credential-helper/ecr-login/cli/docker-credential-ecr-login

RUN go get -u $REPO

RUN rm /go/bin/docker-credential-ecr-login

RUN go build \
 -o /go/bin/docker-credential-ecr-login \
 /go/src/$REPO

WORKDIR /go/bin/
$ docker build -t aws-ecr-dock-cred-helper .
Sending build context to Docker daemon  2.048kB
Step 1/7 : FROM golang:1.15
 ---> e5b4455acc93
Step 2/7 : ENV CGO_ENABLED 0
 ---> Using cache
 ---> 32df66289405
Step 3/7 : ENV REPO github.com/awslabs/amazon-ecr-credential-helper/ecr-login/cli/docker-credential-ecr-login
 ---> Using cache
 ---> 3ff787805cb3
Step 4/7 : RUN go get -u $REPO
 ---> Using cache
 ---> 7117a30722f5
Step 5/7 : RUN rm /go/bin/docker-credential-ecr-login
 ---> Using cache
 ---> 2780ccb85ab8
Step 6/7 : RUN go build  -o /go/bin/docker-credential-ecr-login  /go/src/$REPO
 ---> Using cache
 ---> f848f437fa25
Step 7/7 : WORKDIR /go/bin/
 ---> Using cache
 ---> 58cd6fc9e9f1
Successfully built 58cd6fc9e9f1
Successfully tagged aws-ecr-dock-cred-helper:latest

[codecov]

Codecov Report

Merging #965 (8536597) into main (61b715a) will increase coverage by 5.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #965      +/-   ##
==========================================
+ Coverage   54.47%   59.63%   +5.15%     
==========================================
  Files          25       22       -3     
  Lines        1452     1412      -40     
==========================================
+ Hits          791      842      +51     
+ Misses        594      500      -94     
- Partials       67       70       +3     

Impacted Files	Coverage Δ
pkg/container/container.go	45.36% <0.00%> (-1.96%)	⬇️
pkg/notifications/msteams.go	66.66% <0.00%> (-1.76%)	⬇️
pkg/notifications/gotify.go	66.66% <0.00%> (-1.08%)	⬇️
pkg/container/client.go	14.28% <0.00%> (-0.31%)	⬇️
pkg/notifications/email.go	90.69% <0.00%> (-0.22%)	⬇️
pkg/notifications/slack.go	100.00% <0.00%> (ø)
pkg/container/util.go
pkg/notifications/util.go
pkg/notifications/smtp.go
internal/flags/flags.go	88.65% <0.00%> (+0.17%)	⬆️
... and 4 more

---

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[simskij]

I think this would be a more suitable solution. What do you think? It would allow us to not depend on an old version of go.